| Hi
It must be the season for it. My '01 S500 has started making a whirring sound, which stops as I slow to idle.
I originally thought it was the cooling fan motor, because when I spun the fan it felt choppy. I took the motor out, squirted it at both ends with WD40 and allowed it to soak a few times and when I spun the motor it was then running freely.
I still had the noise though, and I've just taken a torch to it and there is some play in the ribbed belt tensioner pulley, so maybe I'm getting some vibration from that.
Try spinning the fan by hand, if it's not smooth then oil the fan motor bearings.
Also try waggling the plastic pulley on the engine belt as a source for your sound.
Regards
Sidious | 
