The autobrake system applies the brakes on gear up selection on the mainwheels,the nosewheels are braked by a primitive arrangement of friction pads on leaf springs which contact the tires when they are retracted into the wheelwell.I suspect it is the nosewheel brakes which cause the vibration as it is usually more intense in the forward cabin.A 747 49x19 wheel assembly is an extremely heavy item - probably over 250lb,as I know it takes 3 people to get it into the back of a pickup! the amount of energy dissipated in stopping it rotating from Vr speeds must be immense.If you have a look inside the nosewheel bay you will see the black rubber dust ground off the tire during the spin braking.