I feel I'm now a trombone collector since I'm up to five, with my latest one being a 1960s Conn 88H. I was wondering if the slide return spring click/clack is normal? If so, is there a way to quiet it down a bit? Thanks!

Single digits? You're still in the age of innocence.

Springs are never completely silent - you're hitting metal with metal - but if they're especially noisy or scratchy it would be worth pulling them out and cleaning the barrels.

There's no need to be constantly hitting them anyway, just tune longer on the slide. They're supposed to be a reminder that 1st position isn't only one place...