Tapping Noise Continues - am Completely Stumped!

[OK-944]

So I’ve been on the trail of this very aggravating tapping noise…which my car’s PO had also experienced but was not all that concerned about - which sounds like its coming from just underneath the passenger floor up near the footwell, and happens usually only when in first gear...but occasionally also in second, and only accelerating very gently at low rpms. Occasionally the sound will continue after shifting into neutral at these low speeds. If I give it a little gas the sound disappears, as it does while gently applying the brakes. The sound is completely absent for most other driving.

I figured that this tapping would simply go away during my recent rebuild/rehab experience…but nope - still get the tapping!

My most recent suggestion as to what might be causing this is a possible pulsing from the injectors when the fuel pressure/delivery is low enough (as when gently accelerating with little pedal pressure) to allow this - and which can cause a tapping noise when a fuel line (most likely return line) happens to be loosely contacting the car’s chassis in certain places.

So…I spent a bit of time last week under the passenger footwell area of the car - applying extra padding to fuel line areas in contact with the chassis and/or in contact with each other. Also removed the wheel well cover and did the same slight bending/extra padding in that area.

A very strange “cure” to this tapping occurs when I very gently apply the brake pedal - which causes the tapping to cease completely, although the noise usually (but not always) returns when I cease to depress that pedal.

My car is a non-abs model, and the cruise control has been disconnected. The tapping frequency is on the order of 360 taps per minute (appx. 6 taps per second), and the tapping itself sounds a bit “ragged,” like loose metal hitting other metal - as opposed to a more precise “clicking” sound.

At any rate…this continues to have me completely and utterly stumped, and if anyone can provide some possible further insight - I’d be very thankful!

[Jfrahm]

Try disconnecting the vacuum line to the cabin for the HVAC system.

[OK-944]

Thanks! But which line? There are two of these coming through the firewall...one which originates at the heater control valve just behind the engine - the other starting at the brake booster.

Hmmm...the brake booster vacuum hose - might make sense in connection with the tapping ceasing as the brake pedal is applied?

Torrential downpour right now...but when the weather clears I'll start by disconnecting the brake booster to cabin hose - and if this takes care of the tapping, then I'll follow this vacuum line down into the footwell area where the sound originates - and hopefully will see what's most likely causing the tapping. Sound about right?

[thomasryan]

interesting. my first thought was the clutch flex line tapping the brake booster.

Is the vacuum steady? 360 beats per minute times a four cylinder is close to idle speed. if the check valve on the booster is silent when one side of the diaphragm is open to atmospheric pressure, the diaphragm will smooth out the pulses intead of the valve. you might place your finger on the check valve to see if you feel the same rate.

[OK-944]

With the tapping occurring either under the passenger floor or, more likely, up inside the area underneath/to the left of the glove box, with the sound reflecting off the floor - I don't see how a noise coming from the clutch line tapping on the booster would telescope down this way.

But I've also been thinking that a check valve might be involved with this. At any rate...sometime tomorrow or Tuesday I'll try messing with the brake booster vacuum connections to see if I can get some results. Could be something as simple as a hold down clamp coming loose someplace, in association with the a brake booster controlled vacuum line.

[thomasryan]

is there a check valve before the vacuum reservoir can by the battery?

[Jfrahm]

I was thinking the HVAC supply vacuum, not the booster. But whatever, it's trivial to disconnect it and see if the noise goes away.

Good thought on the check valve, not having one could cause all kinds of issues.

[walfreyydo]

Not sure if this is the same tapping you describe, but the heater control solenoid has been known to go bad and create a very rapid tapping/clicking sound which can be traced to under the dash. Also corresponds with heat that wont turn on/off.

Lots of threads similar to this one if you google:
https://rennlist.com/forums/924-931-944-951-968-forum/943321-clicking-under-dash.html

[dme]

I had a tapping clicking sound coming from around under the glove box. Turned out to be my bad climate control unit clicking the solenoids on and off.

[OK-944]

This is definitely not a solenoid-like clicking noise...but more like someone striking a metal plate with a wooden spoon. The tapping is also very "imprecise" - in that the noise level tends to be variable.

Have not yet pulled any brake booster to HVAC vacuum lines to check...but I really hope that when I do this I'll finally know a bit more!

[88Silver924S]

If you're still stumped by this I would check all heat shields on the exhaust system to make sure there is always a gap between the heat shield and the exhaust pipe/manifold. Also make sure the exhaust pipe hangers are doing their job properly and not allowing the exhaust pipe to touch anything that could cause such a tapping. Check that the exhaust pipe is straight where it is supposed to be and bent where it is supposed to be and hasn't been "bent out of shape" by running over a rock or other hard object that kinked it out of proper alignment. Also grab the exhaust manifold and try to move it to make sure it is still bolted down tight. Good luck.

[OK-944]

Found the vacuum hose from the brake booster through the firewall to one of the two air recirculation solenoids (earlier mentioned as "suspect") to be disconnected, just outside of the firewall. Re-connected this and...no dice - still have the tapping! But I'm still thinking that this is a vacuum issue - related to both the braking (hydraulic and vacuum) system (given that the tapping disappears with light pressure on the brake pedal) and the HVAC (vacuum) system.

I'm also wondering if that detached booster to HVAC hose, seeing as how I may have not noticed this after I'd dropped the engine, had been detached long enough to compromise the HVAC system in some way which causes the tapping.

Thing is, even with that hose detached, I still seem to have good and controllable heat, although maybe it could be a little more intense.

As for heat shields, exhaust pipes, etc. - I was quite thorough in positioning and tightening things down as I reinstalled my engine and other components. About the only thing I can think of along these lines relates to a recent reference of tapping associated with the lower clutch cover (that thin plate underneath the clutch and bell housing) needing to have four bolts total (two are typical but there are indeed four threaded holes) to mitigate a tapping noise in at least one case. But again, if this were the case...why would a gentle push on the brakes silence this so consistently?

I'm thinking that my next move might be to remove the glove box (and perhaps the DME computer cover, to give access to more hoses, recirculation solenoids and associated conntections, etc. - and then perhaps have a passenger position a mirror with an upward pointing flashlight so that the tapping might actually be observed as I drive the car.

Thing is...this could be as simple as a detached hold down clamp - but until I can catch it in the act, I still have no idea. About the only reason that this has not driven me completely insane at this point is that the car itself is working fine...and my gut says that, at least for the time being, the tapping is not associated with anything harmful.

[TheSecondChris]

I don't really know much about the cruise control components on the 944, but on most cars when you press the brake it cancels the cruise setting. I mention that because the sound goes away when you press the brake, so might be a possible connection between the sound and cruise control (tempostat in Porsche speak) system. You say that cc was disconnected on your car - was this done by you? Maybe the sound is a result of the exact method someone used to disconnect the cruise control?

[OK-944]

I'd also thought about the cruise control...but with the electrical and mechanical path of this system moving up from the left of the drivers footwell, up and over to the passengers side - where the throttle module sits high up just behind a strut tower...I don't see how any sound from this might telescope downward.

Then again...I'd never really resolved that the cruise control actually worked prior to my purchasing the car (not a high priority, and I got a great deal on the car) - and while the PO basically said that he thought the cruise control was intermittent, I really did not care at the time. I do have a complete history of this car, and maybe I missed something about this so will look it over again. The PO only owned this car for a year before he sold it to me, but the guy who owned the car prior to the PO had it for 25 years, and from all I could see from the history, he kept up with the car pretty meticulously. Hmmm...he lives not far from here - maybe I'll reach out to him for any further details he might be able to share.

Sad story though...after I'd recovered my seats and was reinstalling them, I mashed the cruise control lever (cracking the lower plastic housing beneath the steering wheel) during this process...so it now hangs uselessly - and I went ahead and simply disconnected the cruise control cable at the throttle module, thinking that I wouldn't want something weird (and maybe dangerous) to happen, throttle-wise, as a result of my mashing that control stalk.

But at this point, everything (including the cruise control) even remotely in the vicinity of the noise is still on the table. Thanks again!

[OK-944]

BINGO!!! (and boy, am I ever a fool!) - Pulled fuse #19 (ABS, Brake Lights, and...Cruise Control), and the car taps no more! After all my hair pulling, probing around, padding and re-bending various lines and hoses...its the cruise control!

Funny thing is, I had actually thought about the CC long ago but somehow did not think it was worth pursuing. Also, before I had the brilliant idea to just...pull the damn fuse - I had actually planned to do a very slow drive by, with the hood of the car open...and have my wife watch the cruise control module as I did this! And then..."why don't I just pull that fuse?" Multiple Head Slaps!

But I do need my brake lights...so need to fully disconnect and isolate the cruise control circuit for now - thinking that when the weather warms up again next year...at a point when I'm otherwise twiddling my thumbs, that I may actually attempt to repair the cruise control system...or not!

Finally...electrical dummy that I am - I am seeking some advice here. There is an electrical cable attached to the rear of the cruise control throttle module (the one near the strut tower on the passenger's side)...and I'm thinking that I could either simply detach this and cap it off, or perhaps follow it to its point of origination and disconnect it there. What I don't want is any sort of loose live power feed waiting to cause trouble.

At any rate...I'm open to suggestions about the best way to go about disconnecting/isolating the cruise control circuit - so I can get my brake lights back. Thanks!

[originalowner]

Pull the module out from under the center console, or at least disconnect it. That will leave the connector under hood sealed.

[TheSecondChris]

Which component do you think was clicking? Any chance it's ABS related since it's a combination fuse?