Humming from rear disc

hello matey, iv got a brand new 225 cup on a 55 plate, now wen breaking i get quite a loud like servo/hydrolic humming coming from the rear disc/wheel only on the drivers side tho, this also happens wen on the motor way doing around 70-80mph it humms constantly and if you go round a slight left hand bend the noise gets louder till you slow down, and then gets really loud wen you break again. seems like the alloy is getting very hot as its the only wheel that i cant get clean. ie the dirt is almost welded on.