Engine tapping

Hi,
First of all, it is very good web page. Question is I have mercury sable 1990, 150k miles on it. I use mostly house to work car. I do basic maintainence on the time. Problem is specially summer time on heavy traffic engine is taping and performance gets low. At that time, I checked engine heat, it was little higher then normal but not very high. On high way doesn't have any problem. I recenly changed radiator, and hoses and checked fluids. Is it a engine problem? Thanks again for your help.