Low Hum in Rear

I know the title sounds suspicious but work with me here... 2010 Tundra X-SP w/ 4.6 right at 160K miles. Just got back from the local dealer and they replaced rear hub because there was a low hum coming from the back. Thought it may be bearing, ujoint, or differential but let them diagnose. After replacing hub I took it down the highway to test. Same low hum. Not an RPM issue because it still does it in neutral. When I was there they wanted to charge $350 for rear diff fluid exchange but I told them I would do that. Too easy not to. Not too sure that it has ever been done before I got it. (Purchased at 110K miles) Any other ideas outside of what I've thought of? Going to change diff fluid this weekend to rule that out and because I'll know that's done.

 

At 40'ish mph or so long slolam back and forth can help rule out wheel bearing loading.

My 2010 4.6 has zerk fittings at all of the u-joints that As Found in 2021 when purchased, had never been greased in 100k miles... Unload the drive shaft and pump some grease in there see what comes out.

As for the diff, they can make noise from poor contact patch due to wear. Usually worse under load but not always. Some hum/howl only when coasting, decelerating, or accelerating. Some only power/decelerating, some coast only.

Play with the throttle and listen.

Also, swap the tires end to end and see if it changes.

Edit Allow vs always

 

Rotate the tires if you haven’t already

 

So i put new tires on about 3 months ago. More of a road tire than i had before. Rotated them yesterday at the dealer when they replaced the hub. Checking all seek fittings as well. Thanks all!

 

They have a TSB for axle shaft seals. Is it leaking or do you notice any wet areas under or around the rear rotors? Low fluid in the axle will cause a noise that will get worse the lower the fluid gets.

I realize it isn’t a bearing issue but might be something to consider.