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This is my first post on the forum, although I've been using it to learn a lot of things related to our beloved Mercs.
I've been driving an '05 W211 with the awesome OM648 straight 6 engine for over 5 years now, and I have to be honest, I have neglected to fix a specific issue.
There is a specific whine/whistle sound, coming from the engine bay, when the car is over 1300rpm, or depending on how much I have stepped on the gas pedal. It gets louder with acceleration, its not present while the car is idling or in low RPM with a slightly pressed gas pedal.
The whine/whistle sounds seems to go away only when the car is in regen mode (or what I think is regen mode, as the idle RPM go a little up to something like 800 RPM). That's the only time I get some peace and quiet
As you can imagine, this has been driving me absolutely mad, for at least 3 of the 5 years of ownership, and many auto repair garages have not been able to diagnose what's going on (or have been too lazy to look at it and would focus on other items to fix on the car).
Car pulls strong, it has all its stock features, like the DPF, the swirl flaps and intact PCV system. It could be the turbo going out, but its been at least 3 years, and I have not felt the car not pull as strong as before or some other issue that could indicate a failing turbo, based on the car's performance.
I am finally coming to terms with looking at the issue on my own time, and on the weekend I will remove the air filter box and take a look underneath. One giveaway that something is amiss, is that when I have had the hood popped before, and started the car cold, I could distinctly smell exhaust gas in the engine area.
Forum members, please, if any of you have had something as remotely close to my issue, please let me know. Any tips on fixing the possible issue are absolutely welcome as well.
Forgot to mention, I will record a video, to show the issue. Also, the whine/whistle is way more prominent, when the car is under load. While sitting on idle, its a lot quieter, but still present.
I took it upon myself to start disassembly around the engine bay yesterday. I removed the air filter box, then a heat shield, and found the exhaust runner to cylinder 1 to look sooty on the outside.
Video shows bubbling after I sprayed it with soapy water. Only on this runner, thr nuts were loose. I tightened them gingerly and no more whistle sounds!
Here's the video of the soapy water test:
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