Model: 98 Nissan patrol Ti 4500 Automatic.
Problem: When accelerating, if the engine goes over 3000 RPM i get a whining sound no matter the gear that gets progressively louder as RPM increases, If i level the RPM out at any point over that mark the whining sound dissappears until i accelerate again, I can be in first, Second, Third, Overdrive and doing any speed, but if i go over 3k Revs the whining appears until i stop accelerating even when doing 60mph in second (4.8k revs) if i hold it the sound goes away.
What could be the cause? It doesn't happen in neutral or under any other condition.
My car has 284000 miles.
My car has an automatic transmission.
I think the rear differential: the ring-and-pinion gears and bearings specifically, are worn out (especially given your mileage). Your specifics about the conditions under which the whine is heard are excellent for diagnostics - well done! Since the whine increases with road speed and acceleration, it means the ring gear (or pinion) is loose enough through wear that the power over 3000 RPM, or the power to push past higher road speeds (air resistance) accentuate the wear pattern on ring & pinion gears and bearings in the differential (rear). I would recommend a new ring & pinion gear set, and accompanying bearings be installed. To avoid any unncessary repairs, consider YourMechanic, as one of our mobile technicians can come to your home or office to diagnose the noise firsthand and help you make the necessary repairs to your vehicle. Best of luck.