Clicking from back wheel?

This only happens occasionally. When I accelerate there is a clicking noise what I can only describe sounds like a hammer hitting something, but once I speed up enough it stops and it doesn’t click when I slow down only when I accelerate sometimes which confuses me. At first, I thought it was like a loose mudflap, but then I saw that it was not. I can’t see anything hanging down. It’s just the click happens every day but not every time I accelerate.