I think that the presure on the oil switch is about 7 PSI. It sounds to me like the pipe leading From the oil screen to the pump could have come loose, partially broken, or a hole could have appeared especially since it came on suddenly. If any of these hve occured air could be sucked in as well as oil which will cause it to cavitate and make a whirring sound like power stearing pumps when the oil level gets to low and starts sucking air. To check this out put on a presure gauge because it should read a lower presure than it should normally but you will still get oilflow and some presure. There is one more posibility that I just thought of. If a key has sheared off the shaft could be spinning the gear but not as fast as it should which could make a whirring sound but your oil presure would also be reduced significantly.
If this isn't the problem the only other thing in that part of the engine that could make a whiring sound could be a spun main bearing but that sound should get progressively worse fairly quickly. I hope this helps.
PS Timing gears could make a whiring sound but that would be at the front of the engine.
