I have this 2012 Mercedes C350 with about 100,000 miles on it and lately it makes a strange noise when I start it in the morning and drive away. It t is in the video it starts at the 5 second mark, you can hear it, and you can feel it in the car , it does it in neutral, or in drive ,and not related to engine speed or brake application. UPDATE : The noise is not there when I coast in neutral with the engine OFF ... but as soon as I start the engine, the sound returns . But it goes away after a few minutes of normal driving and when I get to work just minutes away, there is no noise but when I leave for home 8 hours later it is back but goes away by the time I get to eh highway about a minute away. But if I only leave it for a few hours, there is no noise. It has to be cold. Its hard to troubleshoot because it is only there for a short time and seems unrelated to most knows weird sounds in all the videos i watched. Listen closely, you have to turn up the volume as i recorded it from inside the car with the windows up and the recorder on the dash. It sounds like a low frequency buzzing, that does not change pitch with speed or engine rev or gear selection , and like I said, even coasting in neutral, but its not there when coasting with the engine off , but only with the engine running , but it stops when i stop the car .. pressing the brakes at any point of driving makes no difference. I am baffled, please offer suggestions or comments.

video here :

You Tube

You Tube

video here better sound

Failing secondary air pump? Try disabling it, pulling its electrical plug or remove the secondary air injection fuse(F32) and relay(N10/1kP) ( are located on the third bottom row of the front SAM which is on the left engine side)

yeah, its not it, when I stop, the noise stops and when I pull out it starts again. also goes away after about 2 minutes or less depending on outside temp

Related to something that turns off or on after a few minutes. So maybe emissions systems related. Crankcase vent valve??? Evaporative emission purge valve???

I pulled out the fuse for the secondary air pump and still there. I placed a gopro under the hood, click the file here , its much louder.