You hear a slight chugging sound at the tailpipe but only after it reaches operating coolant temperature. The driveability remains adequate and it passed smog. I recently changed one coil.
Pull one spark plug wire at a time (pull on the boot, not the wire itself) and see if there is a change in operation.
If you pull a wire that is damaged or has a fouled plug, you won’t notice any change.
That usually points to an ignition miss - plugs, wires, etc., but could be other things.
Sounds like a burnt exhaust valve too.