Wheel bearings, when badly worn, make the car sound like a spaceship and the sound tends to get higher pitched as you drive faster. Does the sound go away when you turn one way, but not the other? It's almost certainly a wheel bearing if that is the case, and it'll be on the side that goes quiet when you turn that way, so if it goes quiet when you steer right then it'll be a bearing on the right side of the car.
It's almost always the front ones, but as I found out recently it can be a rear one too, although this is much rarer. Labour shouldn't be too much, and the parts themselves are fairly cheap. It's worth finding out if there are other parts that are worth doing at the same time, on my car for instance it's worth replacing the strut top bushing and bearing while the strut is out, so I did those while I was at it. It costs a little more now but will save you spending more money down the road.
To verify that it's a wheel bearing, find out which side it is with the above method, and then jack up the front wheel first, as it's probably the front one. Grab the wheel with your hands at the top and bottom and see if there's any play or wobbling at all. If there is then the bearing is definitely shot, if not then it's still worth getting checked out, but at least you'll know for sure.