GLC250 Squealing Brakes

[Tripz78]

Hello everyone. Im a new Merc owner and i bought a GLC250 SUV from a PI about 4 months ago. My mileage is currently about 6000km and ive this very strange issue with my brakes, specifically i believe its coming from the rear brakes. 

When i drive for about 25-30 mins and i apply my brakes there is a loud squealing sound. But this sound goes away when i press harder on the brakes. But when i apply light to mid pressure it squeals quite badly. 

Heres the more irritating part, this only comes out when my brakes are really hot. As mentioned, it happens after abt 25-30 mins into a drive. When the brakes cool down, there is no sound, but as soon as it gets hot again, the squeal appears again. My rear pads have been changed once at 5000kms (i asked the PI to change to see if the sound goes away, but it didnt)

Im wondering if any GLC bros here have experienced or heard about this before? Would really appreciate some advice as the sound is driving me nuts!  Thanks in advance. 

[Ah Bui]

Get your workshop to take a good look at the calipers...  are all the pins holding correctly? Pads can last you at least 30k... 

[Tripz78]

Thanks for the advice bro Ah Bui. PI workshop is chaning rotors on Thursday ill make sure they check the clips to. Appreciate your replies.