This is a hard problem to diagnose since its related to a heavy load on the pedals, then you can't look for the cause with the bike in a stand because it won't exhibit.
Some things to try:
Test the rear rim brakes (only) by going somewhere quiet and unhooking the rear brake, then try a powerful start and see if it squeals. Then hook your brakes up again immediately.
Do you have mudguards/fenders? Check for witness marks anywhere that might be close to a wheel.
Give your rear wheel a wiggle - it could be a loose axle contributing to some movement.
Frame flex -Stand over the bike and with both brakes hard-on put some load on a pedal in the horizontal position. Look straight down and see how much your frame flexes. This will give you some idea about how much variance it has when you press really hard.
I have a bendy steel rigid MTB with home-made mudguards. Under hard pedalling it make a buzz noise as the knobbies rubbed on the plastic used to make the guards. Turned out there was sufficient frame flex under load to make the rear wheel contact the guards, but not enough to find it in the workstand
Squeals tend to be brakes or bearings because most rubbing noises sound exactly like rubbing noises. A squeal is higher-pitched.