1989 535i Automatic

This has been happening off and on for a year or so. If I stop the car and leave it in drive (D) holding it stopped with the foot brake, as for example while waiting in a long line, sometimes as I pull away I get a metallic sort of rythmic rattle that sounds like it is coming from the drive train. It increases with the car speed. If I stop and shut off the engine and restart it goes away. It only happens after a long idle in gear with the brake on.

Any ideas what the source could be?

TIA

Jim