I found a nice MTB on Craigslist recently, and to get it rideable, I took the wheels off my other MTB, replaced the brake pads front and rear, and went for a nice ride. No brake noise.
Then after replacing the tires on the bikes original rims, I put them on. Holy crap! The brake noise was bad, from both wheels.
I alcohol cleaned the rims, set the brakes with a little nose in attitude and tried it again. Better, but still noisy. What am I missing here?