Bearing or diff stranger sound

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

It sounds like it could be in the rear diff not the bearings normally they make a roaring noise. Have you tried cleaning and adjusting the rear brakes. I would try that and see if it helps

Yaris is front wheel drive car. The noise may be coming out of the rear wheel bearings. This often happens if the bearings become aged or when the water traps into the bearings.
Thanks

http://www.oreillyauto.com/site/c/search/Repair+Manual+-+General/03501/C0008.oap?model=RAV4&vi=1278301&year=1996&make=Toyota Here's a couple of repair manuals,always good to have one so you can trouble shoot on the spot.

you would only experience problems from the diff if there was a substantial increase in torque from the different engine as there would be less than 10% difference in torque ratings , Any noise from the diff would be naturally occurring from wear or oil
diff noises indicating bearing problems are a whine on drive or coasting , an growl on deceleration and a harmonic whine or growl when the vehicle is held at steady speed Other than those indications , I would be more inclined to suspect wheel bearings and brake associated noises
wheel bearings are normally more pronounced on cornering

Possible wheel bearing. Check tire pressures as well.

Sounds like a wheel bearing going.

like grinding sound ??
if so it's your wheel bearing.

noise producers on vehicles include the worn brake pad indicator that rubs on the disc when the pads are dangerously low , loose seal shields on the diff pinions , tail shaft noise from failing bearings , "U" joints , center bearing supports and shields for that bearing touching the support bracket from failed bearing rubber mount
if is only heard when moving ,what I am suggesting that everything that turns has to be carefully inspected
for example a bit of rock stuck between the diff flange seal shield and the diff housing will create a noise that is amplified by the tail shaft

It's the wheel bearing. Guaranteed. Just replaced mine. Go to Audiworld tech articles for step by step instructions on how to test the wheel bearings. They must have been put in wrong last time or they just went out again.