Tried search but cannot find a match to my problem in 2017 E220d AT.
A week ago a very loud buzz/noise started, and refuses to be influenced by pushing anything in the area.
I am pretty sure it is an electrical related problem, and think maybe a small fan in some behind-dash electronics?
Here is why. On a 4 hour journey (with gritted teeth, and loud music to try to suppress the annoyance) it repeatedly would stop entirely on throttle-off glide mode, engine iding.
In this case the speed continues and engine idles, but noise reduced / stopped after about 30s, not immediately. So not mechanical revs but probably voltage drop as alternator volts lower and battery goes off charge.
Also the noise is not affected by bumps and bounces of the road, so its not traditional mechanical. Starts around 60kph changes a bit at higher speed to a more urgent frequency. I have had little computer fans that run a bearing and sound like this a bit.

You experts that know the detail of the behind dash modules, which have little fans that could do this? Display, NAV, stereo amps, voice computer ?
I'm going to try pulling fuses to see what affects it, to try to confirm an electrical link. Air con / fan fully off, still there.
I know even if the radio etc is off on the dash likely many modules are powered, and fans would probably be on.

Would love some assistance on this please! And hopefully might help some others with this problem... Thx Greg

Well indeed it is the stereo / nav. If powered off after its been running I still get the noise. But if powered off before starting (and perhaps car cooled down also) when car starts with it off, there is zero problem.

So again super-keen guys, which modules have the fan that must be the culprit. Some of you will have pulled these units out I'm sure!
Thx, Greg