Loud Humming above 50mph

I have an 07 2.7L manual transmission Tacoma with 180,000 miles on it. Recently I was driving along on the interstate and when I went on to the off ramp and let off the gas there was an unusual deep humming sound which sounded like the engine. As the days went on I noticed that even with the clutch in and the engine at idle RPM the humming was still there and would get deeper as I slowed down and then disappear under 50 mph. Any time I let off the gas over 50mph it appears. Any ideas?

 

I think it might the end diff. Mine did that for a while, I think it stopped or I don’t notice anymore.

 

I would check wheel bearings first, then diff, driveshaft, and transmission.

IF it's a bad wheel bearing, the sound is probably there when you are accelerating, but masked some by the engine noise. Check this thread out:
https://www.tacomaworld.com/threads/how-to-diagnose-front-wheel-bearing-failure-2nd-gen.332548/

 

wheel bearing