Jeff928S4

1989 S4

This 928 has been very good to me........when I had the 944, I posted a lot more than with the 928....but, alas, here is another 928 post.

I've had a plethora of odd "sounds" appear with the 928, but they have usually gone away. Good luck, I guess.

Now, just yesterday, I get in the car and turn the key. I hear a "knock...knock...knock" coming from what sounds like inside the dash...almost like the heater fans or something. Turn the key to the "off" position and try again and get nothing.

Start the car and upon moving forward for the first lil bit, I get the same sound (but it sounds "different"...more like it's coming from the passenger footwell area). It's almost speed-related in the sense that the knocking speeds up with car speed, but then it goes away - it doesn't continue like say a bad CV joint would. Odd.

So, what is "fired-up" when the key is turned? I'm guessing a fuel injector or something - but with my "good" luck with the car so far, I'm not well versed in 928 problems........yet. What could this "knock" sound be?

Th Dude

My guess...
A deteriorated vacuum bellow for the climate control system.
There are six of them under the dash.

Wait for the experts to chime in and confirm.

Jeff928S4

When doing a search on "knocking" noises.....the air pump and vacuum leaks were mentioned. Is there a way to check this?

Th Dude

"Sometimes" it will go away if you place the climate control position switch onto a different setting.

It happened to me. Again, the experts will chime in soon.

AO

Is the car running when you hear this? or just before you engage the starter? Does it change with RPM? How loud are we talking... is it more like a tick or more like a thud?

I assume it was with the car running. If it varries with RPM, I'd say air pump.

Jeff928S4

The initial "knocks" I hear happen when engaging the starter. I'll hear like 3-4 knocks, then it stops. They are quite noticable.....like someone is on the other side of the dash with a very light hammer, hitting the vents in the middle.

Once the car is running, the "knocks" will resurface when the car starts moving. At first I thought it was something stuck to the wheel, but the sound goes away once the RPM get over 3000???

BIMMERMIKE

I have the same problem. I beleive its the fan blower knocking on the shroud, probably due to lose shaft or bearings. If you close the center vents the sound is muffled. Depending acceleraion rate or cornering the sound goes away. Not sure what the fix entails.

Jeff928S4

I forgot to mention that sound also is alleviated when I turn a corner. This had me mega-confused. I was thinking CV joint for sure.

I always have my center vents closed......now I'm afraid to open them and listen.

I've always had a problem the fan making a "whistling" sound when I put it on "feet" only. This summer, the dash is coming apart damn it!

hinchcliffe

Don't the relays tick when they are on their way out? Maybe Fuel pump tick bouncing around in the cabin?

Jeff928S4

I recently went through a failed LH brain, so I'm very familiar with ticking relays and this is not a ticking relay sound.

Jeff928S4

**UPDATE**

The sound in my dash is no longer sporadic....it is a full "knock knock".

...but here is the kicker - it only does it when the heater sliders are set to either "feet" or the mix of "feet" and "defrost" (or windshield). If I put the slider on "defrost" or the "all" (the solid white block on the slider), there is no knock.

Obvioulsy there is a problem with whatever makes the heat goto the feet area.

Are there separate vacuum bellows for "feet"? Are they easy to replace/get to?

I'm off to search!

Jeff928S4

**UPDATE**

The sound in my dash is no longer sporadic....it is a full "knock knock".

...but here is the kicker - it only does it when the heater sliders are set to either "feet" or the mix of "feet" and "defrost" (or windshield). If I put the slider on "defrost" or the "all" (the solid white block on the slider), there is no knock.

Obvioulsy there is a problem with whatever makes the heat goto the feet area.

Are there separate vacuum bellows for "feet"? Are they easy to replace/get to?

I'm off to search!

P.S. - Thanks Randy for changing the thread title. My title wasn't really "helpful" now that I think about it.

AO

Based on this info... it's got to be the footwell flap or a vacuum component thereof. I'm thinking you probably have a leaky vacuum, because over 3k rpms, it goes away (higher vacuum). Search the WSM and the archives and I'm sure you'll find your answer.

the flyin' scotsman

Boyeee...............does the noise rise and fall as you increase and decrease the heater fan setting? I had many noises come from the HVAC blower before it starting screaming at me all the time............the bearings were blown. I was unsuccessful in getting rebuild advice so I bought a used replacement.

The blower is under the plastic cover on the firewall that also covers the wiper mototr etc. Remove it and with the fan on listen for the 'knock' noise.

JP Rodkey

Hard to determine without actually hearing the sound. Based on what's posted thus far, I would disconnect the blower power lead under the cowl and see what happens. I'm going with Bimmermike at this point (fan cage bumping around).