Knock from rear end

2002 325i sedan
very strange situation.
sounded like I had something rolling around in the trunk, crashing against the walls. Then, I thought it may be backfire in the muffler. Stood outside with engine running, could not hear anything. But definitely noise when sitting inside. Doesn't make a difference if the car is standing still or moving. Same noise. Also, this only happens intermittently. Sometimes, a couple of days passes without anything. Then, when you least expect it. Crash. Any ideas. Exhaust, drivetrain, fuel sytem? No engine misfire. At least, not yet. Thanks in advance for any suggestions.