Likely my problem is unrelated but this sounds quite similar to my experience after having the disk pads changed at a local truck garage. The squeak or high pitched rattle was continuous with the brake pedal pushed. Quite noisy, embarrassing in city driving. It was only in one front wheel. The truck mechanic stopped by to look and thought the problem was rust on the rim of the brake disk. He ground the rust off and the squeal stopped until three potholes into the next camping trip.
Once home again, I took both wheels off and had a really good look at the new pads. There was a difference in how the noisy one was positioned compared to the quiet side. The shiny tabs were not placed the same.
I wrapped electrical tape around that tab in the photo and that changed and reduced the sound. I got the mechanic to have another look and he corrected the installation mistake on the noisy side. The tab is securely tucked in so it can't vibrate. No further trouble. This mechanic is a friend and very experienced (mostly on much bigger trucks) so it must be an easy mistake to make. The brakes worked fine and there was no squeal on his test drive or for me until I took it on a camping trip.