odd humming sound when driving/brake issue? VDC slip?

Hi. My 2006 Xterra S has been making an odd sound during a cross country drive (Albuquerque to Hartford, CT). It's mostly a low humming sound that reminds me of a tire with low psi (but it's not - all tires at normal psi). From a stopped position, it kicks in within about five seconds of driving, sounding like "mwummm mwummm MWUMMM MWUMMM..." getting louder as I accelerate. It becomes a steady low hum and stays that way until I decelerate, silencing before I come to a complete stop. After coasting in neutral a few times and punching the gas, the sound does not correspond with RPMs bur rather tire speed (or so it seems). I had to brake hard on a few occasions on this trip and wondering if a brake component didn't get stuck somehow (I haven't seen anything, but I haven't looked all that carefully and am not really sure what to look for). My VDC/slip light has not been on during this time, although it did come on tonight (and it was on a lot over this past winter when we had tons of ice and snow here in CT). I have two different sets of tires on, with the relatively newer set on the rear.

Thanks for any thoughts.

I am getting my '08 SE checked out tomorrow for the same type of sound. She runs rough once I get over 60 mph and can really hear it when I coast and then apply steady pressure to maintain speed. I can't tell if it's a tire issue, engine, or drivetrain. I'll post when I find out the noise....