What the hell is this noise?

Well if it were say miss-aligned teeth in the diff, you would hear more of a howling noise, and it would be constant through either speeding up or slowing down. Easy check for dealership or for yourself if you have a workshop is drain the diff, take the cover off and put yellow paint on the teeth and assemble then free wheel the diff to check for teeth that are too far out of alignment.

Usually if you see marks that are on the outer edge of the gear that will indicate the howling noise I spoke of.

 

It's kinda funny but I have a 2011 Tacoma with only 22,000 miles on it. I am in college again getting my 2nd degree in the Oilfield and I live practically 2 minutes from Campus so I don't drive mine much and I thought I had the same U-Joint problem on mine for awhile I would hear a noise that sounded 100% like a bad Joint and I just let it roll and now I don't hear it anymore.

 

do you happen to have nfab or other type of steps on the side?

I ask because before I put on sliders, my truck came with nfab steps from the dealer. When they mounted them, there was not enough space between the bar and the truck body right behind the front wheel well. If I hit a bump or caused enough body flex, the nfabs would rub against the body.

 

Nope, no mods of any kind yet.