150000 miles. loud poppng noise in passenger compartment

You may have an exhaust system that when it heats up or all the time is collapsing internally. It would be more apparent when warm. An easy way to check is to warm car up then have someone in the car and be at the back and hold your hand near the tailpipe. You will feel exhaust coming out at idle, now have the other person rev the engine and hold the idle up does the exhaust increase. If not you have a clog and it needs to be addressed for safety. I don't think you have a cooling system problem.