I have a 2007 S4 B7.
The other day I turned the car on and heard an unusual hum coming from the back driving side of the engine. I would describe it as a mid range toned humming that sounds like a belt. It matches the rpms as i rev the engine for a second before the sound of the car drowns the humming out.
The sound has gone away twice randomly after the car has warmed up but doesn't always go away after its warmed up. Most of the time it stays the same no matter the temp of the car.
I have taken a small video of the sound with the hood open.
Please let me know what this might be and how much it could cost to fix.