I don't use the 4wd. When I bought it a couple of years ago, it seemed to not
be quite right in 4wd.
I put the back up on jack stands, and when I run it in drive and overdrive it
makes the sound. It could also be described as a helicopter sound. It sounds
like it is at the rear differential, but the sound could be just traveling down
the driveline. It does not make a sound in reverse, first, or second.