Noticed this sound the other day. When I put the runner in reverse and keep my foot on brake I'm hearing a rumble sound. If I take my foot off the brake and roll back in reverse i hear it as well. Have no idea what it is, made an appointment with Toyota for this Saturday. Seems to be making the sound only in reverse. I'll see if i can create a sound clip.
I think it's the pipe above the transfer case skid. I have RCI skids and during cold starts it would rattle against the transfer skid in reverse only when I'm on the brake. The hangers degrade over time causing the pipe to droop when there already isn't much clearance at all from that skid.
Replace the hanger with a poly unit or use some metal clamps to lift up the that pipe. That's what I did and it did away with the rattle/vibration.
Edit*
I did the worm clamp thing twice and after time spent on bumpy roads or trails it will fall off. I've lost two so maybe zip tie it so when it falls off you won't lose it. I also swapped out the OEM rubber mount with a Perrin 12mm poly mount part # ASM-EXT-100. Supposedly the material doesn't sag over time like OEM but after a couple of days it sagged and began contacting the skid plate again.
I used (4) 1/2" 5/16 SS spacers and (3) thick washers for the rear 7/16 carrier bolts. Not ideal as you lose a little ground clearance but if you want to get rid of the rattle/vibration you just gotta lower that skid plate OR re-route your pipe OR re-bend your skid to allow more clearance.
Here is a picture. Don't mind that little pebble stuck in there.
