I fitted 312mm brakes to my vrs the other day I drove about 50 miles then they started humming (like a high pitch vibration sort of sound) that goes away way when brakes are applied i replaced both front wheel bearings about 2 weeks ago so I don't think it's them I checked it wasnt the rear brakes by pulling the handbrake while the noise was there but it didn't make it go away but the pedal does does any one have any idea what it could be?

Might be drive shaft related. Check for any play in cv joints. Also check the discs are running true. Had similar proplems with an old volvo. Used a needle guage to check discs were fitting properly. One was slightly out of alignment but managed to rotate it to get it to align properly.

Looked like this

Doesnt take much on axle stub to put disc off alignment. Mine was only slightly out and couldnt feel it through brake pedal. The larger disc may be causing vibration if slightly out.

Alasdair