2001 Honda Civic - loud humming noise on freeway:

I have a 2001 Honda Civic sedan - over the past 2 weeks, I have noticed a loud humming noise when I am driving at higher speeds. It almost sounds like I have a prop airplane under my car! I have put my car in neutral at the high speeds to see if it might be an engine thing, but the sound still occurs and changes pitch depending on how fast I'm going.
I have about 88K on it - my brothers all think it is the wheel bearings, but my dad questions that because of the fact there is no grinding or knocking noises.
Anybody heard this before and can help so I don't end up taking the car in for repairs and getting ripped off? The more I know the better!

Thanks!