i have a 89 Bonneville with the 3.8l that is making a metal on metal sound, it sounds like it is coming from the engine internally. it has no problem starting up and running, i just don't let her run to long cause it sounds bad.I checked the harmonic balancer and it is still good along with my alternator. I also replaced the water pump thinking that was the problem but nope and as far as what i have heard it is the oil pump so was wondering how difficult it is to replace, thanks for your time.

Its rare to see one of these oil pumps fail... Possible yes... Chances are no...

You could have any one of several issues..

Could be a Bad rod bearing( worst case )
Could be a Bad Harmonic Balancer ( Dampener )
Could be a bad balance shaft bearing and the list goes on...

Can you get a video of the issue?

The first things I think of when I hear a loud metal squal is the water pump and alternator. You could remove the belt and spin each pulley to feel for binding and listen for squeaks.

Also, you can run the engine and try to listen for the approximate source of the sound. The harmonic balancer will usually make a thwump thwump sound. With the belt off if you try to wobble it it should not move. If it does, then its bad (the rubber between sections has separated due to age at this point).

If you spin the alternator and it squeaks, then the bearings are shot and its bad.

If you spin the water pump and there is any back and forth play, binding, obvious fluid loss (look for a green streak) or squeaking then its bad.

If the noise is only present with the AC on then the AC compressor is bad or the clutch has issues.

_________________
-Eric
"Energy and Persistence Conquer all Things" --Benjamin Franklin
-1989 Buick LeSabre T Type: 220k miles
-1987 Fiero GT: 224k miles
-2012 Honda Civic (boring, but its a good DD)