I've tried replacing the supercharger with a rebuilt one which helped - but the noise came back in full force shortly after. I've heard it could also be the supercharger pulley, but I haven't been able to find one aside from ebay - any suggestions on where to look?
I have no idea if the bearing is a stand alone item. Generally, you need to spend $500.00 to get a $2.00 part, which makes sense to the manufacturer, but not to the consumer.
Also, as I am sure you know, the clutch drive is an electromagnet, and it could be the clutch plate rattling when it's engaged. Strange it disappears at higher RPM. There is a space tolerance on the plate when it's disengaged, and a feeler gauge can tell if it's the right space, but I don't know what that would be. I imagine it's a searchable question on the net.
Being the same principle as an air conditioner clutch, an air conditioner repair shop could probably check it out a lot cheaper that a dealer, if you have the space tolerance.
Anyway, good luck, and let us know the end result.
That noise is the supercharger coupler. It is a non-metallic composite that connects the clutch driven pulley to the internal drive of the supercharger.
It sounds like the rebuilder either did not use the correct coupler and it prematurely wore out or it was not replaced during the rebuild and is failing. Either way you will have to remove the supercharger, remove the pulley and disassemble the front case to access the coupler and replace it. The actual job takes about 20 minutes (remember to install the correct amount synthetic oil after installing the front case). The supercharger remove/install takes a couple of hours.
The Eaton M-45 series is a relatively common supercharger and you can google the replacement coupler "kit" easily for prices and availability. The intake, discharge and pulley are M-B specific pieces, but the supercharger itself is an Eaton M-45 series... the same as used on GM supercharged engines (think 3.8L Buick V-6's).
These are all aftermarket parts since M-B does not consider the supercharger as a serviceable part.
Thank you for the responses! It looks like the s/c coupler is a cheap fix so I will start there and see if that takes care of the problem. BTW, for anyone looking i've found a couple of places for the m62 coupler:
My car has been making this same sound at startup for a couple of years. It happens almost every time I start the car in the morning for the first time but thankfully (knock on wood for luck) only once at startup. When it is cold outside, it often does not make the sound.
Please let us know how your project goes and please post videos if you tear your unit down.