Depending on the noise, I'd guess at possible exhaust leak or worn CV joints/wheel bearing if it's coming from the front. If the noise fluctuates when you pass over a slight dip in the road or when going around corners, it's most likely related to the front drive system. I've had this slight rumble before and it was hard to isolate until it got worse and I could tell which side it was coming from. Turned out to be wheel bearings once, and CV joint/axle another time on the other side.
Front wheel bearings are generally about $200 or so per side to do at a shop. CV joint & axle is about the same labor but figure another $90 or so for the parts. Rears should be under $100 if the noise is coming from the back. Exhaust leaks are usually $20 worth of welding unless you need a new muffler or pipes.
