My 1993 190E has started to make some nasty rattling noises

I noticed it tonight after dropping off a friend and my door was open. the car was idling in D with my foot on the brake and there was a loud rattling coming from under the car.

put the car in neutral, and the rattling went away. put it back in D and rattle came back, but if I apply throttle while holding left foot on brake, the rattle goes away.

does anyone know what the source of this is? I also have a 1989 300E W124 that does this same exact thing.