C180 Benz W202.... Noise when car is IDLE in D position
 

Hiya..I recently bought 96 model c180 with auto transmission.. Car is in excellent condition overall. I have noticed one rattling noise when car is in D position and brake is pressed. as soon as i accelerate the noise vanishes. Is it cause of the low rpm or what? when car is in N position its absolutely normal but in D position at there is this rattling. Its very similar when you try to drive a manual car in second gear from idle position. Can some one help me on that..
Best regards.. Mallhi