Exhaust popping, could be coolant?

I think folks are thinking of the slight vacuum that occurs after an exhaust pulse passes and is moving on down the pipe; the same. But I would think that is pretty much non-existent at idle. If it was coolant, though, it ought to go away pretty fast as it dried out. It the headers are still being run open the popping is normal in my experience and will continue as long as they are open.