Have 2000 forster with rear humming noise. Both wheel bearings are new stil r/r diff same problem.

Gear oil won't make the humming noise, lack of it will. Depending on type of vehicle you're talking about.

A howling noise from a diff is an indication of the gear mesh( pinion to crown wheel) is not correct. Several reasons being worn pinion bearings--loss of preload on the pinion bearings --- crown wheel carrier bearings failing ---not sufficient preload on the bearings---wear on the teeth from incorrect diff oil in use. or simply wrong diff oil. The grade should be 80w90 OEP for normal or special oil for limited slip diffs.

check side carrier bearings in the diff i have rreplace a lot of them for the same noise thats all you have to do remove the diff cover remove end caps and see if the bearings are pitted if so replace

ok it could be two things. it could be rear wheel bearings or a rear diff. If its a high pitch wine then its worn gears in your differential. its needs serviced or replaced. If it more of a rumbling or grinding hum, coming only from one side or both) then there are bad wheel bearings.

for the humming check the rear tires for uneven wear. if the are choppy that will couse a humming noise. next check the axle seals, if they leak the rear diff will run low on fluid and cause the bearings to make noise. if the seals are leaking the rear diff will have to come out and be inspectec for internal damge,last possability is the rear wheel bearings they can be checked by raising the vehicle and moving the tire in and out to see if there is slack if there is movement from in and out they will need replacement, the have to be pressed in so you might want a shop to do that repair

Hi, there, Check your break pads. Could be from there.
cheers

Possible wheel bearing. Check tire pressures as well.

Sounds like a wheel bearing going.

Check the wheel bearings

probally brakes they can rust a bit