I think out 96 2.5 Outback needs a new oil pump.  we freshened up the engine with new bearings (ACL racing bearings off ebay) and new gaskets and whatnot.  When you first start it up, you can hear the bearing rattle a little, then it goes away when the engine builds oil pressure.  everything seems to be at book spec, which the haynes manual says 14 to 26 psi at 2000 (Fully warmed up)  It does about 30 psi.  but when you let it idle, it drops to 4 psi.  would a new oil pump solve this problem?

Did you actually mic the journals or just pop new bearings in it?

On the first start did you prime the oil galleries before peri the engine fire? You're supposed to pull the plugs and crank with no spark or fuel until the oil pressure light goes out, takes about 20 seconds.

 

Oil pumps almost never go bad on these, but the screws on the backing plate can get loose and allow the backing plate to separate from the housing. This is fairly obvious if you have the pump off.

 

Either way, if brand new bearings are making noise you probably need to pull it apart and replace them again.

we didnt mic the jounals we just plastigauged the bearings.  .003 on all mains. for the rods, .003 on number 4 and roughly .002 to .003 on the rest

I think you can find pump clearance specs in the FSM. Pull the pump and check it just to be sure.

How many times has the engine been run since the rebuild? Have you driven it at all?

Main and rod bearings are not forgiving of low oil pressure, especially on the first start after a rebuild. It's not good to let the engine idle during the first start. You must keep oil pressure up for about 20 minutes while checking for leaks. This flushes away any material that wipes off of the bearings and allows the bearings to form to the shape of the journals. Plenty of other factors lead to premature bearing failure. Cleanliness being one of the most important. Any dirt or dust between the bearing and journal is a problem. Dirt between the bearing and caps is a problem.

Not sure how common this is, but if the bypass valve on the oil pump is faulty it could cause low oil pressure. The pump itself is pretty simple, a gear/cog style positive displacement pump. It seems like the heck would have to wear out of the gears before it would trouble. The backing plate screws right, those were often a problem too.

 

I had trouble a couple years ago with oil pressure after changing a timing belt, valve cover gaskets and some other things. It turned out to be the oil filter. On a whim I tried a new oil filter and that solved it. The filter on there I had just put on when doing the timing belt. I cut open the old filter to see if it was defective. The filter was fine but there was this black sludge junk plugging up the filter. It was strange, the problem never came back. I think the previous owner didn't change the oil often and on this oil change along with the timing belt and other work is dislodged a lot of crud all at once and plugged up the oil filter.