The 747 uses pretty fast airborne speeds. What you are hearing and feeling is the wheels all spinning at high speed. During the retraction sequence of the main undercarriage, brakes are applied automatically so that debris is not thrown around inside the undercarriage bay. You should only hear this near the wheels- in just a short distance away in the cabin, it will be inaudible- that is why other people and the crew are unconcerned- they didn't hear it!