High pitch whine/wheezing sound from climate control

[jkg]

I've recently purchased a 2007 CLS (W219), which makes a high pitch sound when the climate control is on.

The sound comes on when the engine is on, and persists no matter if the AC is on or not. The sound does not change with fan speed, and disappears completely if the climate control is turned off. The sound does not appear when running the climate control with the engine off. The sound does not seem to come from the blower/under the passenger well. It sounds more like it's coming from under the center dash, but it's hard to tell. All vents are fully open.

I can best describe the sound as a high pitch whine with a very slight wheeze to it. It's accompanied by a faint lower pitch sound that sound a like a belt drive or similar.

As mentioned, the sound is persistent when the climate control is on, but it will occasionally lower in intensity, or disappear completely for a second or two - often when the car is unsettled, such as during or immediately after a sharp turn.

The AC blows cold air. The wheeze reminds me of prior cars with low refrigerant levels, but two things are throwing me off. 1) It's not just a bubbly liquid-y sort of sound, but accompanied by a high pitch whine, and 2) the sound does not change with the AC being on/off, or the temperature set to cold/warm.

Any ideas what it might be, or how I best determine the source?

 

[noogieman]

Could be the bearing in the fan motor making funny sounds

 

[jkg]

Could be the bearing in the fan motor making funny sounds

The one in the passenger footwell? Doesn't seem to come from there, and the noise is only present when the engine is running.

 

[Dodipops]

What you are describing sounds like low refrigerant pressure. Worth having it checked I would say.

 

[jkg]

What you are describing sounds like low refrigerant pressure. Worth having it checked I would say.

I agree. But the sound doesn’t stop when turning off AC, so I’m not too sure it could be that?

 

[exvolvoman]

I have a similar sound which I've narrowed down to the blower motor, it stops when fan & a/c turned off, as yours is noisy when engine is running could it be a noisy bearing on one of the pulleys?
Air compressor or alternator maybe?

 

[C240Sport97]

Is it the temperature sensor unit on the roof?

 

[noogieman]

The one in the passenger footwell? Doesn't seem to come from there, and the noise is only present when the engine is running.

Yes, that's the one.
Bearing could whine or have a glitch.

 

[Sprint-Man]

Good afternoon and welcome to MBCUK!

Sorry to hear about your problem and maybe I can assist?

1) Is the noise louder inside the engine bay - Around the fuse box?
2) If so, remove the fuse box cover - Is the noise louder?
*) My theory is that a small auxillary fan (used in cooling the AirCon condenser - I believe?) located in that area is simply dirty, or more accurately the fan impeller blades are dirty?
**) It may be possible to vacuum clean the fan whilst in situ, or remove 5/7 torx screws and remove the fan/clean/refit?

I do hope you find a solution, good luck

 

[jkg]

Thanks for your hints everyone. It helped me a lot to determine the source, or at least rule out the potential sources not related to low pressure.

In the end, turns out it was just a refill that was needed. Asked the mechanic, and while he wasn't sure about the CLS (ATS, so no merc expert), he told me quite a few models run the AC continuously with the climate control, no matter if you have AC enabled or not. So I guess that could have been what was throwing me off.

Simple fix in the end. There's still a slight faint of bearing noise in one of the fans, so I might have to look into that at some point. For now, it works well enough to just avoid specific fan speeds