Post stated the problem goes away when setting in neutral. If the noise varies in relationship to the speed of the car and perhaps the load on the drive line. Has anyone considered U-joints? Worn diff bearings?
If this is strickly related to the engine RPM, then I agree, think oil system related.
Suggestion. Get an oil filter cutter and inspect the oil filter for metal shavings. If you find any, you will know the answer.