So here is something interesting i've noticed on my few months old W211.
When the cabin a/c or heating is not on, everything sounds normal.
The minute i turn it on and weather the a/c compressor is on or not i notice whining sound from what i can describe the area behind the steering wheel.
It changes with the rpms change. It is very faint but i hear it.
I thought it was the transmission but again if the fan is not on it's not there.
i removed the fan from under the glove compartment to clean it and looks like new and there it normal fan noise when on.
I then looked at the air circulation fan in the upper console and i can hear it running as a normal small fan, no whining.
then i tried hear the fan on the right hand side of the steering wheel opening and it doesn't sound like it's coming from there.

Is there another fan somewhere or inside the engine bay?
has anyone experienced this?

It's almost impossible to capture it on audio so you have to take my word for it.

Appreciate the feedback.

 

If I recall correctly, there should be a fan at the back of the COMAND unit, as well as the motor for the CD/DVD player

 

Correct but the noise was there even with the head unit out recently for some sirius connection.
It's more inside the dash OR leading into the engine bay.

 

Perhaps looking at the parts diagram for the AC system, and understanding the location of the moving parts

Sedan: http://mbepc.net/1/fg/e/65h/211056#gsc.tab=0
Wagon: http://mbepc.net/1/fg/e/65j/211256/0/83/#gsc.tab=0

 

It's it's not the fans. You're hearing the AC compressor through the lines. That's why the pitch changes with engine RPM, not fan speed. Turn the AC off but keep the fan on, wait about 3 minutes until the AC compressor is completely (it doesn't switch off instantly). Behold no more noise. Every time you start the engine even if the AC is switched off your AC compressor is running that's why you're hearing. Wait a few minutes with the AC switched off and the noise goes away. If you hit the big O button in the center of the climate control unit the noise goes away instantly because the AC compressor switches off instantly along with the fan. My 04 does it too.

 

ok
i will test this later.
what is the big O in the Center console? Is that off?

and if this is the case with the a/c compressor what is the fix? New compressor?

 

Off/On for the compressor. It is both. Basically, it does the opposite of what it currently is.

I would not jump on a new compressor so quick.
1 - Check codes for AC system
2 - Measure pressure on both sides,: high and low, using the appropriate procedure (discussed in other threads in the forum). Are they within the specifications?
3 - Perhaps worth reading https://www.mercedesmedic.com/diagno...mercedes-benz/

 

Ok, checked for codes and all is clear.
Your logic holds water on being the a/c compressor.
I will use statoscope to confirm.
So, looks like might be the a/c bearings (not the clutch) and wondering if i should tackle that next.

 

One person in a previous thread mentioned it might be too little refrigerant in the system. To be honest I've never checked my pressure. The AC blows ice cold in mine.

 

Hey, I'm experiencing the same issue and it's driving me NUTS. The AC system was completely emptied yesterday, checked for leaks and brand new fresh refrigerant was put in. There is still that annoying whine behind the dash the moment I switch on the AC or blower. As soon as I hit the 0 - it goes away. It changes with RPMs so I was leaning towards bad AC compressor - mechanic told me mine is all good and he is almost certain it's something behind the dash.

Did you fix yours?

 

the A/C is ON when the light on the "0" button is OFF, and the A/C is OFF when the light on the button is illuminated (on).

 

I get a strong whistling noise if I shut the rear air vents. The fan overpowers the front vents and they start whistling. When I open the rear vents, everything quiets down.

Peter

 

Your mechanic has it right. The compressor bearings inside fail and start whining. I have the same exact thing happening and have already purchased a new compressor to replace mine with.

 

The noise from the AC BLOWER (behind the glove compartment) is a VERY COMMON issue on these cars. You can bite the bullet and buy a new blower, or attempt to lubricate its bearing and see how long it lasts. Up to you. Your compressor should be fine as your mechanic stated.

 

Hi gogydm,

Please let me know if it fixes it for you.

After my last visit to the mechanic, when I last posted, it seems that the whine has reduced, but definitely hasn't gone away.

 

I definitely agree with you, but why would the sound change with RPMs?

Also, last year I took out the blower and got a professional shop to change the bearing and some kind of guides inside, as it was whining. So now I am confident that it's not the blower itself. Perhaps the blower resistor could make that whining sound? Someone else suggested that, but how would I test it, aside from buying a brand new one?

 

RPMS of the blower are a function of the blower voltage regulator which is also sensitive to alternator voltage. Our 2008 E350 had that annoying noise that increased with RPM. I lubricated once, and it was ok a few months, but it came back. Replaced the blower 5 years ago, and so far so good.

 

But you replaced the blower AND regulator, or just blower? And what brand did you buy?

 

just the blower, the regulator was fine. Still the original @162K miles.

 

Yes, but given that @dnikolov-211 said that the noise goes away when he turns everything off, and therefore the computer commands 0% AC compressor, is leading me to believe that he has the same issue as I do.

I'll report back after I replace mine.

 

I think you can double-check by unplugging the blower, and activating the AC for a few minutes and see if the noise is back or completely gone. A system of many parts must be split into manageable parts to diagnose subsystems in isolation.

 

I am not sure if the AC compressor would engage if the blower doesn't spin. I have to check Xentry if blower RPM is reported.

 

I am almost certain it won't, but I will try.

@JCM_MB Where is your sound coming from? I feel like on my car it's coming in from the centre of the dash.

Also, the whine starts to be more noticeable one the system was on for some time. For example if I drive 10 mins to a store, it is almost unnoticeable. But if I got for a 1-2 hour drive, then the whine/humming becomes very annoying inside the cabin.

 

You are describing the exact behavior my car is exhibiting.

 

I can't believe I have FINALLY found someone with the same issue!

I have chatted with some people on Facebook groups and most of them resolved this whine by topping up refrigerant, which didn't actually work for me.

Well since you already got the compressor, I will wait for your feedback. If it's not that, I'll buy a brand new blower and report back.