2000 camry loud humming sound starts 55km and 110km but car runs fine what could be causing this?

when driving under 55km per hr car is silent and runs fine but as it reaches between 55km and 60km per hr a heavy humming sound develops then dies away, then as it reaches between 110km and 115km per hr it comes on again and goes away and stays away if i stay above 115km per hr while i am driving on the freeway. can someone tell me what could be the problem? Help.                   Thanks in advance.