Mechanical noise in dash on trailing throttle

[911fool]

'79 SC with 80 miles on rebuild: I've read many entertaining dash noise threads in Search but have not found anyone describing what I am hearing. Only on trailing throttle I hear what sounds like a mechanical clacking in the instrument area, as though I have a mechanical speedometer or tach. Goes away under throttle. The noise is there regardless of speed but only when I let off the gas. What's turning in the dash?

[911fool]

The noise may be my throttle linkage. I have not yet installed carpet or door panels so I'm hearing things I normally do not hear with a completed interior. There's enough play in the linkage that I can rattle the rod with my finger, sort of replicating the sound which does seem to be coming from the dash, but maybe not.

[911fool]

I'm talking out loud to myself on this, I guess, but the noise is not the throttle linkage. This is a steady clacking sound that sounds like it's in the dash only when I lift off the throttle. I can't feel anything in the instrument glass or when touching the dash. Sounds louder toward the driver A post. Could be in the trunk?

[TRE Cup]

Check the vacuum hose connection at the power brake booster. Temporarily disconnect it, plug the hose to the chassis and go drive the car
CAREFUL since you will not have the vacuum assist !!!!! So take precautions to leave more space to stop and use more pedal pressure to do so. Listen to see if the noise is still there

[911fool]

Interesting thought. I'll have to wait a few days to do this, but will. Thanks for the suggestion, and I'll report back.

[911fool]

Originally Posted by TRE Cup

Check the vacuum hose connection at the power brake booster. Temporarily disconnect it, plug the hose to the chassis and go drive the car

I tried this and the noise remained. I do appreciate your reply. Guess I'll keep listening.

By the way, here's a photo of my fresh engine, now with 150 miles. This is my first engine build. Club members (Bluegrass Region) assisted.

[1980SC]

I'm dealing with what may be a similar noise in my SC. It's driving me nuts! At a steady throttle input or idle there is no noise, but when decelerating in gear or full throttle I hear a high speed tick-tick-tick-tick-tick from somewhere behind the gauges.

My car also has PMO carbs.

I was certain it was coming from either the tach or the speedo, but the noise continues with them both removed. I've unplugged everything I could find behind the dash, and nothing stops the noise. Turn signal relay, master cylinder connections, all the relays on the fuse box other than the fuel pump relay. Nothing stops it. It's a euro car so no cat mileage counter. I even plugged the vacuum line for the brake booster.

I thought it might be ignition related so I've tried new plugs and wires. I swapped in a spare coil and tried several old distributor caps and rotors I had. The tranny ground strap in new. Nothing helps. I need to order a new cap and rotor and try that next.

[911fool]

Today we found the tapping noise. It's the brake booster check valve. Squeezed the rubber hose near the 90 degree fitting on the back side of the booster and the noise got louder, could feel the pulse from the engine. Richard Parr is sending me a check valve for my booster hose off the PMO manifold, but I'm not sure this will stop the noise. If the noise remains after installing the new check valve on the engine, I'll try removing the check valve at the booster.

[911fool]

The second brake check valve, installed inline off the left intake manifold, stops the noise at the brake booster check valve!