Squealing rear brakes when not applied

So I am having a small issue with my 2004 1.4 new beetle cabriolet. I thought I had a sticky piston on the drivers rear, so I replaced the rear calipers as a pair was only £65. followed all bleeding procedures and the brakes feel really good. Plenty of meat on the discs and pads cleaned all hardware and greased everything up with copper grease, and the slide pins with white grease. But I am still getting the same issue. A squeak happens while normal driving and stops when the brakes are applied. Any ideas what I may be missing here?

Thanks in advance
Sean