My own guess is that you’re just getting very normal brake vibration. Brakes just naturally vibrate and unless things are done to reduce/eliminate it or the noise they will be noisy. Did you apply any kind of brake grease to the backing plates at the contact points?
The thing about time & temp probably just has to do with rust build up. Surface rust builds up very quickly. This makes noise as the shoes grind the rust off, and it also increases vibration. So when you tear them back down again, use a quality brake grease to lube the backing plate wherever the shoes might make contact.