Engine RPMs drop around 60 mph

I’ve got a 72 El Camino with a 4.8 and a little cam. Trans is a 700R4 (about 400 mi on new rebuild). Nothing crazy, but a fun daily driver. When I get it up to higher RPMs (tachometer not installed yet) and hit around 60 mph, the RPMs drop really quick and I hear a sound like a rattle can in the engine compartment. As soon as I drop back down in speed the engine picks up again. Thoughts?

Edit: I’m just starting out with working on the car myself. Bought it from an old timer who’d done a lot to it. Would really appreciate any tips or guidance. Here to learn.