Weird noise when going over rough roads

My daughter has a 2002 Toyota Camry V-6 with 121,000 miles on it. When going over railroad tracks or hitting bumps in the road the car makes a rumbling noice. We believe it's in the back. We've had two different mechanics look at it and they can't find a reason for the noice. The idle arm was replaced a year ago because a mechanic told us that was the reason. The struts have been checked and we were told that wasn't the reason for the noice either. We've had all of the bolts we could find tightened, the muffler system checked to see if that was hitting when hitting rough roads. Any other ideas? I myself have a 1999 Camry 4 cylinder and don't have this problem.