Thwacking noise at 2.5-3.5 revs. What is it?

Hey everyone. First time poster! I've got a noise I can't identify. As the title suggests, it's a 'thwackthwackthwack' kind of sound. It happens in any gear between 2.5 and 3.5k revs but not when accelerating, only when cruising.

Any suggestions? Thanks in advance