W202 C240 Humming from Rear

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New Member
Apr 15, 2014
W202 C240 Sport
Hello all. I've recently picked up a lovely old W202 C240 Sport. The only thing wrong with it at the moment is a bit of a headscratcher. The car intermittently makes a relatively loud humming noise with strange symptoms, and it sort of sounds electrical. It's one of those annoying noises where I can't quite pinpoint where it's coming from.

Firstly, the noise sometimes happens as soon as I start the car (including while stationary) and remains for an entire journey with no change. Sometimes it only occurs when the throttle is applied. It never changes pitch with one strange exception - if I'm not moving so everything aside from the noise is quiet, I can make the pitch change slightly by raising or lowering the windows.

Does anyone have any ideas? I'm pretty confident that it can't be anything drivetrain related as it happens while not moving, and my gut says it's not coming from the engine bay. Definitely seems like something electrical but the only thing I can think of is fuel pump - don't know why raising or lowering the windows would impact this though.
Heat shield? They have a habit of corroding around the fixings and dropping down.

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