Hey guys,

I have this loud hum/whirring noise on cold start which seems to go away when engine is warm. Lasts about 30 seconds. Car is a 2004 L430 with 152K miles and the timing belt, water pump, and seals were done by previous owner at 95K. What are your thoughts? Maybe serpentine belt related bearing or belt itself? Video is below. I scheduled an appointment with a mechanic which is highly recommended on these forums (Elite Service Center) but the earliest I could get is January 11.

 

Loosen the serpentine belt, remove it, start the engine, and see if the noise persists.
If it goes away, then it's gotta be one of the pulleys/appliances driven by the serpentine belt. With the belt off, you can try spinning each bearing by hand to check for noise or play.

I had bearing noise that would go away when warm. I knew it was my alternator since my voltage was dropping as low as 12.6 at idle, but my trusty mechanic's stethoscope also pointed to the alternator.
No more bearing noise after I replaced the alternator.

Good luck!