mike77

After performing some recent work on my car (1990 s4) I have noticed some noise coming from the dashboard.

It is a little hard to explain. It is not very loud and only really noticeable at idle. It is intermittent and does not always occur. I can't really pinpoint the location, it seems equally loud from the centre console or from the door vent, I think it is more on the middle /driver side (rhd). It sounds a little like a paper party horn and is constant, when shutting the engine off it sounds like it is deflating/powering down for a second or two.

All the work I did was in the engine bay.

Hall sensor replaced
Air pump plumbing removed
Vacuum line and airbox plugged
New oil breather pipe
New cam cover gasket
Waterbridge refresh

I guess perhaps I need to look at vacuum stuff behind the dash as per dwaynes writeup. Just wondering if there is anything else I might consider?

Tom in Austin

My blower motor made a light, constant sound at lowest setting. You might try unplugging it and see if that makes the noise go away.

Another noise you can have under the dash is a bad vacuum actuator. People say this can make a brief 'powering down' sound when you turn the engine off.

kurt_1

^^^ THIS!

I had something very similar and it turned out to be a leaky vac actuator diaphragm. Don't remember which one, though.

You can diagnose it with a Mity Vac checking on the different outputs of the actuator solenoids in the center console.

Kurt
__________
90 S4 satin black metallic

mike77

Ok thanks guys. The powering down sound seems to be a bit of a clue. I'll have to plan a bit of trouble shooting.

You fix one thing and something new crops up.

davek9

Could also be coming from the "inside" temp sensor that's mounted in the console (the little round hole with the grill), there is a motor in the sensor that pulled cabin air through it.

I have one that makes a faint noise when G force is applied (hard turns/braking).

Adk46

I have something similar, making a sort of slapping sound, like a leaf stuck in a slowly spinning fan. But it varies according to engine vacuum - fast at idle, slowing and stopping with opening the throttle. Apparently, some actuator is wonky. Mildly annoying - it's going on the snow list.

NC928S4

I had assorted vacuum actuators noises and whistles followed by running out of air whoosh sound when turning engine off. I swapped in 4 new actuator diaphragms which all had pin hole leaks but that was only part of the problem.

The real culprit was a crack in the rubber manifold that connects to the HVAC solenoids. A used replacement was $5.00 and was a relatively simple repair compared to the diaphragm repair job.

mike77

I think I need to wait for it to happen again and try harder to pinpoint the noise. It sounds like it could be a number of things.

Other thing I forgot to mention was I had the air filter housing off and also finger punched a hole in the maf mesh. Wonder if a noise could come through to the cabin from the engine bay.

Have a feeling it is related to something I have done as I only noticed it after putting the car back on the road.

WallyP

The "party horn" sound is often the little vacuum check valve next to the brake booster. When you have a vacuum leak in the HVAC (usually, but not always a leaking vacuum actuator), the air leaking into the vacuum system will cause the rubber and spring check valve to hum, whine or shriek.

The real problem is not the check valve - it should not have a significant amount of air flowing thru. My suggestion is to test each of the vacuum actuators, and block off the line feeding any that leak until you get your round tuit and get the actuator replaced.

mike77

Thanks for pointing out the brake booster valve. Next time I hear the noise I'll pop my head in there and see if I can hear anything from it. I also wonder if I may have dislodged anything around there. I had to move a bunch of stuff in that area to get the cam cover off.

This write up from John Pirtle looks like what I'm after to do some tests.

http://members.rennlist.com/pirtle/svc_act.html