I just got done tracking down a engine related rattle in my M111 powered 2000 C230 Kompressor. The rattle was most pronounced in the 1200-1400 RPM range under light throttle conditions, it would disappear at higher RPM ranges but would reappear at 3200-3800 RPM with with reduced volume. The noise was low pitched, low frequency growl - I finally tracked it down to the A/C compressor. The rear mounting bolt for the compressor was finger tight and the compressor was vibrating against the mounting bracket - I tightened it down and all is well. If your symptoms are similar to mine check your A/C compressor mounting bolts.