Sounds like bucket of bolts

'97 SC2, 120k miles, 5-speed, getting 120k maintenance tomorrow
When backing out of driveway (happened only once), sounded like something heavy and metal falling in trunk - trunk is empty.
Anyone have any ideas?

Pull up the carpet and look where your spare tire goes, maybe something came loose around there, your jack could have flopped around or maybe a nut that is supposed to hold the tire came loose or something.

Since I was the last one to be in the trunk and use the jack (jack is stored in well with spare tire), I did not think anything would be loose - there wasn't.

Does it happen everytime you back up?

Does it happen if you roll back in neutral ?

maybe you park brake was slightly seized???