I just finished some maintenance on my dad's 4Runner (2007 V8), and I noticed a rumbling noise that was never there before. It happens in reverse around 600-700 RPM. The noise is there whether I'm stopped or rolling, on or off the brakes. It doesn't do it in Drive, Park, or Neutral, only in reverse. The noise seems to be coming from the engine bay. The engine was mostly warmed up when I figured out how to reproduce the problem, but it happened a few times before that.

Here's a video: https://youtu.be/c6WjO_c9-1k

I don't think any of the recent maintenance is related. I changed CV axles, replaced the front differential fluid, replaced the coolant, bled the front brakes, replaced the rear brake pads and rotors.

It has RCI aluminum skid plates on the engine and transmission, so there's a slight chance one of them is vibrating, but the bolts are tight.

Any ideas what could be causing this noise?

It turns out it was the skid plates. The catalytic converter guard was just barely rubbing on an exhaust heat shield in one spot. A quick trim with the reciprocating saw made clearance. In addition, the plastic spacers at the back of the engine skid had fallen out, and there was a pebble trapped between the frame and the engine skid.