P38 Squeak from Rear Wheel

Hi My 2001 P38 has developed a squeak which sounds like it is coming form the drivers side rear wheel. When driving at 30mph approx, you can hear it, if I apply the breaks slightly it stops. When i let off the breaks it starts again. [New pads all round fitted 6 months ago and new disks 12 months ago]

any help appreciated.

I had a similar issue with the Hippo - bit of crap had stuck between the disk and the disk mud guard