humming noise, U-joint?

I've heard plenty of wheel bearings, and it doesn't sound like that. Wheel bearings are higher pitched and this noise is low and deep. To me, it sounds like it is coming from the center of the car. I don't think it's the transmission because taking it out of gear makes no difference... I hope it isn't the rear end because that was rebuilt less than 20k miles ago.. so that leaves the U-joints. Guess I'll have to crawl underneath and see if I can get the driveshaft bolts loose one of these days. The previous times I had to pull the driveshaft, the driveshaft bolts were rediculously tight from the transmission shop.