2001 Dodge Caravan Can't figure noise

I just found this site & some great questions & answers. I can't seem to diagnose a noise problem. When the car is running and in park the engine sounds brand new- hardly any noise. However, I notice this sound when I take my son on his paper route during stopping & starting. When I am stopped with the foot on the brake it sounds like a rattling noise (almost like a diesel engine sound) coming from the front passenger side (under the hood). When I put the car in Park the noise disappears, and when the car is in drive there is no noise. The noise only appears when it is running and in drive with the foot on the brake. I took it to a garage and he said that he'd have to take a closer look but thought maybe the alternator. Any ideas? Thank you so much!