I am getting a harmonic noise from the rear brakes once they warm up I can drag the park brake and this silences them for a bit but it comes back recently had new pads and rotors put on rear could it be a lack of shims?

 

Who was it that did the work ??

You might want to jack it up and see if there is any play or noise in the bearings.

 

Had that same issue a few years back. New shims with a liberal coating of anti-squeal compound fixed it!

 

If you can pull the wheels and calipers yourself it would be a cheap stab at it. Might be the pads are lightly rubbing against the rotors and vibrating causing the noise and shims could cure it.