sometimes, usually cold and humid nights, when I release the brake from a stop, it lets out a long downward whine. A friend thought it was a vacuum leak and upon release the air goes in the hole and makes a noise when the humidity is just right.
I don’t know how else to describe it other than, back when it first started happening, I would look around and open the windows thinking it was a siren (just the downward part)
I’ve had parking brakes do that, but never the regular brakes. For your problem, I think something w/the brake power booster is an excellent place to start. The hose between the intake manifold and the booster could be partially plugged with gunk, the connections on either end of the hose could be getting loose, the hose could be collapsing, the check valve as Tester mentions above, or the diaphragm inside the booster could be leaking. If you connect a hand held vacuum pump to the booster, does it hold vacuum to 20 inches hg?
How cold, @jjustinmbirchb? If below 32* you may have ice accumulating on the vacuum vent valve. But regardless of the temperature and ice the vent valve is a likely cause for the whine.