My 96 made a tapping noise at idle (as well as the ticking of the valves) and I had a slight knock between 1500-2000 rpms. Always had these noises from the time I bought the car w/ 40k miles to the time I sold it with 170k. I maintained the car by the book, still runs great today... F-series engine is just noisy. Even my new K-series isn't particularly quiet at idle. Don't know what else to say without hearing it for myself..