What's causing metal squealing coming from back brakes?

Knowing that it needs brakes, I'd look at the condition of the drums/rotors and make sure that the pads aren't grinding a groove that would cause them to need resurfacing. From there, look to the caliper and check for signs that its getting ready to give up (worn boot for example)

Do u think it's a caliper clip hanging and then releasing

That is one possibility, something you could check on while ensuring the safety of the vehicle

The squealing might just be the warning tab on the brakes telling you it is time to change the pads.