1998 GM LS 130k

i hear a metal clunk when shifting only into reverse. I believe its located in or near the transmission. It is loudest after being driven and then shifting into reverse. It seems that if the parking break is used before shifting into park, then letting the car sit overnight, the first shift in reverse in the morning (before even shifting into drive) is the quietest. If I then drive the car and later shift into reverse, its a much louder clunk. Any ideas? Bad trans or motor mount? Thanks