Just put a new tube and tire on my back wheel. While pedalling everything's fine, but when I coast down a hill there's a sort of whump-whump-whump happening, as if there's a lump in the wheel. The wheel is laterally and radially true.

Could this just be a bad tire or tube? Or am I heavy enough that the valve connection to the tube is coming into contact with the road? Thing is, this didn't happen before I changed the tire and tube, so I'm stumped. Any ideas?