W202 strange noise fron heater/AC
 

Hello list, good day ! Great site, lot's of info. This is my first post since joining a year ago. Was wondering if anyone can help me out with this. The other day I noticed a very faint thud/thump from W202 with 54k when the climate control is on. The sound is so soft that I can only hear it with windows closed and total silence. The sound seems to come from behind the console close to the passenger's foot well. It's very faint and comes at an interval of every 5 seconds. It only appears in heat mode and not in A/C mode. The climate controls works wonderfully except for this noise. It disappears when I switch off the climate control or when I slowly lower the temp setting until the flap moves to cool air. BTW, I have the early pushbutton control panel, not the DIN type.

Not sure if this is related, but I guess I have vacuum leak somewhere because the vacuum pump in the trunk cycles itself 4 to 5 times after starting the car. The leak is very minor as I can lock/unlock the doors even after two weeks of not starting the car.

Question: Where could the noise be coming from. flaps ? solenoid valve ? It is normal ? Can I just be hearing things because the heater flaps are open ? Any long term damaging effect if left as it is ? Thanks in advance !

Willy

Definitely sounds like a vacuum leak to me, especially if your hear the vacuum pump running several times after start-up. Best way to find the leak is to remove the glove box and right under dash panel to gain access to the vacuum manifold for ACC vent controls. Carefully disconnect and test all lines for leaks with a vacuum tester. ACC vacuum line schematic will be needed to know where leaking lione goes if you find one, but it sounds like it is probably the footwell flap vacuum element. These elements can be a bear to replace, but not impossible. Good luck.

Thanks for the quick response. The vacuum leak was my initial suspect as well, but then if that's the case, wouldn't it constantly make the sound in both heating and A/C mode and not just only in heating mode ?

Due to time constraints and lack of funds, I might have to wait a while before taking the car in, will this cause any damage to the system ?

TIA,
Willy

I noticed that my car does this as well, but I have no problems with it and it has always made this barely noticeable noise since brand new, so I always thought it was normal.

But now, you have me wondering what it is. Maybe someone could shed some light on exactly what causes this noise? It also makes this noise when using the heat on the rest mode when the engine is off.

Wouldn't suspect any problems in the short term, but every time the vacuum pump under the rear seat runs it gets a little closer to the end of its life(very long term effect). Reason for noise only occurring when heat is on: that is the only time footwell flap vacuum element is activated if you are running the ACC on "auto".

On my car, I have the noise that elegance is referring to, but my vacuum pump does not run. The car has made this noise since brand new, so if it is a problem, it was there when the car was built.

Thanks R.M. Purselle for letting me know that I still have ample time to take care of this problem, what a relief ! Regarding the footwell flaps being open in heating mode when set in "AUTO", the funny thing is that it continues to make the noise whether in "AUTO" or "EC", and regardless of any vent setting. The moment I switch out of heat mode, the sound disappears whether in "AUTO", "EC" and any vent settings. Strange !

The gentle thumping...?

My 1995 C36 did it.
My 1998 C230 does it.
Dad's 1998 C230 does it.
My neighbour's 1998 C280 does it.

My car has done it since new.

The noise you hear is normal, but I would have the additional running of the vacuum pump checked. I've seen some leaks associated with the rear-head-rest release feature on the C-Class and the W210 E-Class.

Thanks John, now that's a great relief ! I'm almost positive that the leak is not from any of the doors as none of them "whirrs" or "whines" when locking/unlocking. As for the vent flaps, it responds properly to the the ACC panel setting. Will definitely check the head-rest-release, should have done that when I removed the rear deck panel when I replaced the speakers.

Very old post, but my 1998 C230 does it as well. Is most noticeable when in "Rest" mode with the engine off. Sounds like a very slow drummer... I am guessing it is doing some temperature mixing or something.

I am getting a cyclical whirring after startup for 10 to 15 seconds occasionally from the back package shelf, that stops immediately if I set the climate control to "O". Am thinking it's related to vacuum somehow & plan to investigate/troubleshoot further.

What year, model of car?

My 97 C280 makes the noise described in the first post. Sounds like from the glove box area. Sounds like a heater flap is cycling back and forth.

I had this issue in my 1995 c36 earlier this year and then it got worse to the point is was screeching occasionally. I just have a vac line blocked off for now but with it cooling off I should figure it out so I can get the heat working. It is pretty simple to diagnose and because it is vacuum related it can make irritating noises as long as the the car is running.