Humming at ~45 mph, not needle bearing

I have a 2018 SR5 4wd with 99xxx miles on it. Recently it started humming at ~45mph. Always right at the same mph. Stops when I hit 50, stay gone to upper speeds, then returns on the way back down when I hit mid 40s. Gone after low 40 mph.

I assumed wheel bearing and so replaced that this weekend. Humming still there. Looks like I have a spare now.

Got to reading about the needle bearing last night and drove in 4wd this AM. Humming stays with 4HI engaged.

Any thoughts?

Looks like there is a TSB for an issue with the rear diff. I'll have to let wife drive and see if it may be coming from the rear and resonating. That TSB describes a groan/whine. This is more of a low hum.

https://static.nhtsa.gov/odi/tsbs/2019/MC-10164329-9999.pdf

I've searched a few times and can't find anything similar, so sorry if this is a double post.

 

I know the Tacomas can experience a rear diff howl, but not sure if we get it too.

Here's a thread about a similar issue:

https://www.4runners.com/threads/rear-end-noise-5th-gen.3542/page-3#post-435190

 

My rear diff whine is pretty apparent that it comes from the back. It’s more of a high pitched “ringing”, not a groan, not a low hum. For me it occurs at a very specific speed range, irc 64-68mph. Not super loud, but consistently audible when on the throttle lightly. Sound goes away completely when you lift throttle and coast.

In your case, I’d go through the basics. Look at your driveshaft/ujoint condition. Grease ujoints and slip yokes. Check the slip yokes for some evidence that they are moving in and out. I think it’s possible for weird noises to happen if they are sticky.

 

Thanks for the responses. I did the drivetrain greasing a month or so back. The hum was there before and after.
I'll try changing differential fluids as a start.

 

I had a similar whine develop to what you are describing at 25-35mph. Ended up doing both front wheel bearings, plus the needle bearing chasing it to no avail Mine ended up being a "abnormal" wear pattern on my KO2's (tire shop said tires were fine, alignment shop said it's the tires) that completely went away with a new set of tires. How many miles do you have on your set?

Check all your fluids just to be safe (Diffs, transfer, transmission).

 

The tires are <10,000 mile Michelin LTX mud/snow. I have an old 4runner with newer take off wheels/tires that I can put on temporarily to eliminate that variable. Thanks for the thought. If that's it I'll be highly frustrated.

 

Bakebon....any resolution to the humming at 45 mph? I think I have the same issue as you. Replace rear diff with a 2020 diff...change front and rear diff fluids, greased driveshaft, replaced needle bearing, balanced and rotated KO2 tires, checked U joints (seems solid), etc....and the humm is still there (even in 4H).

Checked wheel bearings...did the shake and the spring test...seems solid and no vibrations.

Next, I think Im going to jack the entire rig up on jack stands...get wifey to rev the rig to 45 mph...and crawl underneath to listen where the hum is coming from.

 

I’m posting at Goodyear for 4-wheel alignment. As I mentioned, I have the same issue.

Goodyear tech says he checked my wheel bearings and driveshaft…and it’s all solid. He thinks it’s the tires…some minor cupping.

Going throw on the snowflake stock wheels/tires later to see if it’s truly the tires.

 

Did you figure out what it was. I am having a similar issue.

 

Was the tires...minor cupping caused humming. It actually starts at 30 mph....then gets louder at 40 mph to 60 mph.

 

Well, that’s good. Mine was the front drivers side wheel bearing.