2001 4.0 engine noise

I have a 2001 4.0 with engine rattle at idle. Goes away when you slowly rev it up. Oil pressure gauge goes to zero at idle. Dealer said it could be timing chain guides. I removed the pan cover, everything clean, no guide parts in the pan. How tough is it to replace the oil pump? I'm begining to think the oil pump is the problem.

I have a 1992 4.0 that cratered at 36,000 miles. Rod came loose and block was replaced. 13 years later I can hear a slight crank noise sometimes but the engine has 157,000 miles now. Fords are known for that type noise in some of the 6 cyl. engines. Before I replaced the oil pump I would use a quart of Marvel Mystery oil in the engine to clean any clogged oil ports. I have no idea how tough replacing the oil pump is.