Totally out in the weeds, but try turning your A/C compressor off after the noise has stopped. Hitting the A/C off button doesn't do it on my E320 CDI, you have to actually turn off the climate control. With the compressor off, it makes a noticeable rattle style noise, but as soon as the climate control is turned back on it stops immediately. I have an electronically regulated A/C compressor though, so when it's "off" with the climate control on it runs at 5% or so I think.
I think the only repair is to get a new compressor.
