Hi Megan,
I'm sorry that no one appears to have an answer to your question. I looked and 102 members have read your posting so we know it's been read.
I would suggest that you talk to your dealer and have them ride with you so they can actually hear the "humming" noise you hear. The noise could be one of many things, and that is why it's so hard to come up with one particular fix. If you don't have a dealer, find a reputable independant mechanic. I strongly recomend not using something like a Jiffy Lube, or Firestone, or Midas, or any other quick fix "advertise Cheap" type of vendor. You have a Lexus, not a Ford. (Don't mean to be a snob) ....it's just a luxury car like Lexus is built to give superb performance and longgevity. It needs to be cared for.
Good Luck. Come back and let us know what you find out.
Paul