Popping in exhaust

Question for yous guys. There is a fairly loud pop coming from the driver's side exhaust pipe. It only happens after the car is warmed up and only at idle or slightly above. The pop gets more frequent after I put the hammer down and then return to idle (in the stoplight relay, for example). It's not loud under the hood, only out the tailpipe and it's only ever happened on the driver's side.

Any ideas?