1986 3.2 Carrera, engine whistling / hissing sound when throttle applied
 

Tried to do a quick search but with very little luck - so apparently a hissing / whistling sound developed when throttle is applied, occurs at low, around ~2k rpm. In the video, engine is at running temp, idle is stable, everything else is seemingly normal - no other strange sounds.

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Any hints where should I look first would be appreciated.

I can hear it, if it's not an air intake leak then it may be the start of a bearing failing in a pulley. It seems to make noise when you goose it then goes away? That makes me think it's more likely a bearing.

First thing I’d do is pull the AC compressor belt if that’s still on just to eliminate that bearing.


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Yes, the sound is very brief and quickly goes away. Also, it ia not present if throttle is pushed gently.

By pointing to pulley bearing, would you mean crank, alternator or A/C compressor pulley?

if you would put back on the original filter and the filter housing you would not likely hear the air...
Also have a look on the left side for the outlet for the brake booster....
here is a link what to look for
http://forums.pelicanparts.com/porsche-911-technical-forum/939566-where-vacuum-port-brake-booster-carrera-3-2-a.html

Ivan

Thanks. Will have a look at it as well.
Air filter and box cover would also have been my first guess, but I already have them on for some time, and the sound came just recently. The sound is now noticeable from the inside when driving, so it probably wasn't there after I mounted it.

ok,have a look at the suction to the brake booster

Ivan

one more thing , are your 2 x AFM hose clamps tight?

http://forums.pelicanparts.com/uploa...1683564930.jpg

On my 85 911 the vacuum line for the brake booster is located just to the right of the cruise control pod behind the driver side shock mount. Mine was pulled lose when I did a partial drop on the engine to R&R the pulley seal. I did not notice a noise, just the brake pedal was really hard to press to stop.

Do you mean these two? Those were tight.

http://forums.pelicanparts.com/uploa...1683570926.jpg

Checked all of the reachable hose clamps, did not find anything suspicious..

Thank you for you input. Redone this area, as I had disassembled this piece during the winter months to go though my cruise control system, to take the vacuum actuator out. Also, nothing obvious here..

The noise very obviously coming coming from the passenger side of the engine, so the search continues..

no those 2 ..the big clamps on the rubber booth

Ivan

I had a nearly identical whistle when the o-ring on my idle speed adjustment screw dried out allowing a bit of unmetered air to whistle past it . Easy to check (Pic here https://forums.pelicanparts.com/uploads18/Idle+Adjustment+1285910043.jpg ) by covering the screw/hole with your finger while trying to trigger whistle. Also easy to replace the o-ring by 1) carefully observing the amount of rotation necessary to turn it (clockwise) until fully tight, 2) turning it counterclockwise to remove it to inspect/replace o-ring, 3) replacing it by turning it clockwise until tight and then, 4) backing it out by the amount observed in step 1. Mine's been whistle-free ever since I replaced my o-ring with one from a kit I purchased at ace hardware -- you can also buy a replacement idle control valve with a new o-ring https://www.pelicanparts.com/More_Info/93011091900.htm?pn=930-110-919-00-OEM&SVSVSI=587&DID=41767

+1

I noticed your oil cap is missing in the picture above. Put that back on and run it again. See if the noise goes away or reduces.

The cap is in place in the video

Sound a little like air to me. I would go through all intake hoses including the big one to the AFM, check for any cracks in any hose and the afm connection. Also the rubber between the 2 intake manifolds. Start spraying some of the intake and vacuum hoses as well as the intake manifold to rule out its air. Some say carb cleaner, some say starter fluid, some use propane.

Start making a list of everything people suggested and go down it one by one.

Did you remove the belt to the AC to get that out of the pictre, Could be alternator bearing also.

Check the hose on the back of the air box that goes to the charcoal canister, its right behind the air cleaner. Mine had a similar whistle with a cracked lid. Smog place wouldnt smog it with a repaired cover so i got a new one and found the tube loose fixed that and replaced the lid, whistle went away,

Do everyone a favor once you find the answer, please post your findings for the future owners with a whistling 911.

Agree - this topic comes up every so often. Be nice to hear the results...

Mine does the same thing. Cup mod to lid with K&N filter.

I took a drive with factory lid in place....waddaya know!? The whistle stopped!

Cup lid back on, there's that whistle again!

I hope yours is the same, good luck!

So. Some life has happened these couple of last months, hadn't time to spend a whole day do troubleshoot all at once. Thank you everybody for your suggestions where to look.

First, I talked to couple of mechanics, and they both pointed they fingers at the non-stock air filter and the Fabspeed air box cover frame, instead of the stock cover. This would have been my first guess also, but I noted, that the sound appeared much later after I installed those.

Anyhow, first think, I put back the stock cover, the sound was still there, but much less audible, and almost inaudible from the inside of the car, when driving. I am sure, If I also put the stock air filter in, the sound would be totally gone. I also serviced the air filter, washing and spraying it with oil, and the note of the sound slightly changed. So my theory is, as I didn't oil the filter when taking it out the box when installing it (it was oiled from the factory), the oil might have dried out in some time, and the sound became more and more obvious. So this case is closed.

As a side note, I was very surprised, how laggyier the engine is to respond to the throttle input with the stock air box cover in comparison to the frame only. http://forums.pelicanparts.com/support/smileys/eek3.gif Did not have to think twice to put the frame back instead of the cover.

Thanks again everybody, and until the next one :)