Oil circulation problem?

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. 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!

First I would hook up a oil pressure gauge and check oil pressure at idle.

How often is oil changed???

I need to do some research but it seems to me I read about oil slug problems.

Look at the Free Online Service Manuals thread in this form, it will give you a idea of how involved a oil pump change is. If and when you do change oil pump change your timing belt and water pump at same time.

How many miles on engine and vehicle?

ANother thing it could be is camshaft, rod or main bearings.

I would not run engine alot to keep from doing any further damage to engine compotents.

If you PM with your email address I can send you some more helpful info.

hope this helps