one exhaust down-pipe hopping

[93300E]

Well, here is the sequence of events, related? I dont know. This is on a 93 300E 2.8. First my belly pan fell off in the back and the screws were gone. So I removed the whole pan until I could make it to the dismantlers for new screws. The next day a truck drops a retread and I go right over it. Soon I notice rattling when the car is in drive or reverse, especially when the a/c is on. Putting in neutral or park quiets it, as does going at a good speed. I put it in drive with the a/c on and put on the e-brake to have a look. One of my down pipes was hopping away, carrying on until both pipes were hopping by the time they got to the tips. I inspected as much as I could without a lift and everything seems to be there and tight. Possibilities: something was knocked off or loose by the retread, or the occurance of the noise and the incident is simply a coincidence. Could bad motor mount(s) cause this? am I doing harm driving her without a belly pan? Has anyone had a similar problem with a pipe rattling?
Thanks for any help,
Brooke