My 1988 VDP V12 5.3L got overheated due to the failed fan switch. Coolant temperature went up to the red zone before I noticed. The next day I was able to start the car with no problem but got very loud tapping noise at idle from the front end of bank B. If I give a bit gas, the noise will decrease a bit but not going away. It seems the noise comes from the cylinder 1 or 2 area. So I pulled the spark plugs out and check the compression. It seemed OK as all cylinders got roughly the same readings around 185.

I thought the noise was from the dropping valve seat(s) due to overheating, but the compression testing results seemed not supporting this. Is there any chance that the compression can still hold with the dropping valve seat? or if any other things can cause this loud tapping noise (it sounds just like a tapping valve but mush louder, like the noise from a car first started after being parked for a long long time)

Any comments and/or helping tips are very much appreciated!