It sounds like stress on the tendons/muscles/ligaments, caused from the motion you're using while putting the DB down.
Either:
-Find a different way or motion to put the DB down, such as using both hands, or put the DB on the bench rather than the floor, or turn your wrist in another angle during the motion after finishing the set, etc. Try different things, only after which if the problem's still there it's likely the exercise itself.
-Use a different exercise-IMO DB preacher or spider curls are at least as good if not better.