1999 N15 sss squeak in reverse

[Sonny.]

Hi all, hoping someone can point me in the right direction. I have a 99 N15 sss hatch that has started making a strange squeak or squeal from the rear somewhere but only in reverse?? It doesn’t happen straight away but after 2 or 3 meters, for instance when reversing out of a car park. I’ve had a quick look under can’t see anything jumping out at me disks and pads look fine just thought someone may have come across this before. 

 

Cheers