Hey mechanics, I need some help diagnosing an odd noise related to my rear brakes.

I was riding in this morning and had to squeeze the rear break harder than usual. I heard & felt a very strange squealing sound in my top tube & stem. I looked briefly and the pads are toed in a reasonable amount, the wheel is very slightly out of true but no big spots, and all the nuts & bolts are tight. The rim is not great, but the braking surface is fairly smooth and clean.

The only thing wrong that I see is that the top tube cable housing is a bit shorter than it should be. I distinctly heard this squealing noise in the stem & front of the bike, but only when I really mashed on the rear brake!?!

Resonance.

https://www.bikeforums.net/bicycle-mechanics/227097-brake-squeal-my-wits-end.html

It can be very hard to eliminate completely. Cleaning up the rims and pads with scotch brite would be the place to start.

Check your spokes and wheel bearings. I've had both cause squealing.

Thanks, that's exactly what it sounded like -- resonance. I'll start with cleaning the pad and rims, then try some other solutions.