Hi, the first thing I would do is remove the spark plugs, to remove some of the load on the bearings, then remove the oil pressure sending unit, then connect a mechanical oil pessure guage, and crank the engine. Make sure you have at least 11.5 volts in the battery, so you don't crank (turn) the engine too slowly. The noise you are hearing could possibly be from the hydraulic tappets loosing their oil pressure due to a pump failure, or pump drive shaft failure as stated in a previous reply.DON"T start taking things apart until you isolate where the failure is!!!It could be something as simple as a plugged oil pump screen, which would be seen when the pan is removed. You could try blowing compressed air back through the two ports that are exposed after you remove the oilfilter, with the plug REMOVED from the oil pan. The oil pan on one of those beasts is not as easily removed as the money from your pocket will be if you pay someone else to diagnose this problem, mate!!