Worn crankshaft bearings are the number one cause of low oil pressure in an engine.

Of course, you can also have: A worn oil pump, a faulty oil pressure relief valve or low oil levels.

One quick and easy way to determine if it is worn bearings is to pour a bottle of Lucas Engine Treatment in and see if your pressure goes up. Make sure you don't exceed the normal level of oil when doing this as this can lead to real oil pressure problems.

Anyway... After you have added this to your oil (or any other brand, I just like the Lucas products) you can watch your oil psi and see if it goes up. If it is worn crank bearings this treatment will increase your oil psi. This will not stop the knocking noise unless it is very slight, so don't depend on this to cure all your problems.

But since the A/C seems to be bogging down the rpm I would suspect that you might have a restriction in your ac system which is causing your ac pressure to be high. Or it could be the clutch is worn to the point that it is dragging.

Try treating the oil and running with the ac off and see what the oil psi does.