Click click click. Or if you are running certain cassette hubs; CLICK CLICK BASTARD CLICK CLICK CLICK, or even CKCKCKCKCKCKCKCKCK.
Some people seem to like a really loud click when they freewheel, some people hate it.
Personally I hate it, it’s the ultimate give-away to security guards and police. If you need to sneak about to get to a street spot then having some fucker’s cassette hub making super loud clicks that can be heard two streets away is kind of a bummer. On the other hand I really can understand how some riders find it a useful indication of their speed. “Click… click… click…”; too slow. “CKCKCKCKCK” too fast. “Clk, clk, clk,”; just right… But is there anything you can do about it either way?