Wrapper

The sound came back and I took the truck out on a hill, shut it off and
coasted. The sound continued but I believe I was better able to locate the
sound to the lower center left of the dash.

This could be it. Upon close inspection I also noticed the needle on the
speedometer moving ever so slightly with each sound.

Of course that could be because the speed is fluctuating slightly because
something (like a brake pad) is wearing on the discs.

Urgh!