Reason is often main wheels outer balance, they can shake a little before spin-down brakes stop them as gear is stowed away after T/O.
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In the perfect world, balance is maintained by wheels being stopped at a random point during gear up operation and so touch-down wear and tear is spread over many points of the tyre, so the wheels/tyres are kept in balance during the time fitted to the aircraft.
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However random is random and tyres can get outer balance, plus if spin-down system is not perfect, wheels can tend to stop in a similar location after gear up operation, means touch-down wear and tear may often be the same place, this will get them well outer balance and can get heavy vibs until wheels are stopped during gear up.
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Seem to remember wheels are braked by alt-anti-skid during gear up ops, if system a bit crook, some wheels tend to stop at common places and lose the random effect, mix that with number of cycles and vibs can follow, all fixed when new wheels/tyres fitted, however apart the vibs/noise no need to worry, the 744 gear is most strong.