Oil pressure issue?

My wife called me and told me the oil light was blinking on and off and the valves were rattling in my 1995 nissan quest van 3.0 . I told her to add a couple qrts of oil and get it home. When she got home it sounded like a diesel truck and had 6 qrts of oil in it.
I drained the oil and put 4 qrts back into the engine and it was not rattling at idle. Tried to drive to work the next morning and got about 3 mile before the valves were roaring again and the oil light was back on.
Is there anything other than the oil pump that could cause this? Is the oil pump as hard to change as it looks?
Thanks in advance for your input!