Gear train noise

[Rawze]

if it is a medium pitch sounding sort of steady gear whine that goes up and down in pitch with engine rpm, almost always, it is the oil pump going out. - I can hear that distinct sound from across a parking lot at the truck stops on every truck that has a weak or failing oil pump, it is very common.

The point where you replace it, is when it is just about half as loud( or louder ) than the engine at 100 feet away or so.