It does sound like something with the brakes to me. Especially since there was recent work on them.
If not a loose bolt then I’d guess a missing brake pad clip. Probably the one(s) at the rear of the pad.
Clips are “springs” that are in front and rear of the pad That help keep the pad from “clacking” back and forth.
On hard acceleration the pad would be pushed forward from the friction. In reverse there’s nothing “pushing” the pad forward, (if it’s missing), so no clacking.
Also it should quiet down when actually braking.
Aha. Would that also cause the rub/drag I experienced today, Do_Tar?