My daughter has a 2008 Totyota Sienna she bought used and just loves it with one exception. Everytime it is backed up the brakes squeal!!! Only during backing does this occur. I’m not a mechanic no do much of that type thing. I find no other problem with driving the van and the brakes work fine. My concern is; is she doing any damage not getting the brakes fixed or is this just a nuisance and can be ignored. My concern is for her and her families safety.
You might have someone inspect the brakes for worn brake pads.
Toyota’s have wear indicators on the brake pads. And what these do is make a squealing noise when the brake pads become worn. Sometimes they squeal when backing up, when going forward, or not at all.
I just replaced the brake pads and rotors on a 04 Avalon. In this case the wear indicators made no noise. So they wore one of the brake pads right down to the backing plate onto the rotor, destroying the rotor.
Tester