I lost my original thread but I wanted to give an update to my problem.
It was titled "air pump". I want to thank all that replied. My problem was that I had a knocking sound every time the engine would get hot after a long ride in the very hot weather and the knock would go away when the engine cool'd off.
I have a '90 Grand Marquis for those that hadn't seen the thread. I was going to flush out the coolant and replace with new coolant since the thermostat was new. I went on Saturday to do so. I went to a radiator/air conditioner only shop which is in Coney Island Brooklyn, for those who know the area it is Larry's Radiator.
While examining the radiator before flushing it a crack was found in it. It has a plastic radiator. It was the kind of crack that didn't create a leak but was more like a squashed in crack closing off some of the radiator from operating. Being plastic it could not be soldered and boiled so I replaced it.
The car is running so cool now even in the 90 degree heat and with the a/c on the highest the temp. needle goes is half way. There has been no knocking since I replaced the radiator but I really haven't driven far until today. I am taking a ride to Atlantic City, approx. 120 miles each way and I have a good feeling that the problem has been resolved.
Again, thanks for all the replies.