It could also be the timing chain tapping the cover. There's a tensioner that is pressurized by oil that can be under-tensioning the chain, but it's essentially a cosmetic "problem" in most circumstances. My Duratec has made that sound since I got it about 25,000 miles ago. I recall Terry Haines saying that it can be fixed, but it's something that you'd want to pull the engine for, and in normal use it's not really a big deal. IIRC, you'd have to start the engine at full throttle to make the chain jump timing.
