Noise from engine - SLK 230k (2002)

I have a SLK 230k Auto, got just over 64 000 miles on it and when I am in drive (and moving at low speed) I get a strange "rasping" sound from the engine lasting around a second.

There is no noise in park, neutral or if I rev the engine, only when I am in drive. The noise returns every 5 seconds or so until I get past 30 mph when either it goes away or I now have enough road & wind noise to hide the sound.

Any thoughts ..