Wheel bearing? Loose lug nuts? Is the alignment out? Put it up on a jack and give the tire a nudge up and down and left and right, or take it to a garage and have it looked at.
If you think there is an issue get it looked at right away because any small issue can turn into a bigger one if ignored long enough and create other problems. No matter what it is, have it looked at tranny, engine rpm, suspension, brakes etc., whatever it is you could save yourself a lot of hassle and money if you get it looked at ASAP.