I have recently had this same problem two different times in the last few years and there is one of two possible causes that you can work with without too much difficulty.
First, there is a little plate on the back of the dryer and in that plate is what is called an Idler Pulley. That pully flexes as the belt goes around it and it is on a pivot that is metal with a little plastic wheel that spins with the belt. You can easily (slips right off there if you take the belt off the wheel) replace that Idler Pully assembly for about 18 bucks and it is an easy do-it-yourself. That is one possible fix and takes about fifteen minutes to take the plate off, replace the idler pully and put it back together.
The second possible Do it yourself fix is that there is a little rod on the back of the tub that turns in a little bearing area (holds the tub in place back there). Unfortunatly, you don't have an easy way to get to them, unless you take the tub out.
However, around that area, most dryers have little vent holes around that bearing area about a half inch from the center. They are just about the size of a straw and large enough to get some lube in there to the center of the bearing area. If you get something like white lithium in a spray can with the little plastic tube so you can guide the spray, you can lubricate that bearing area without taking the tub apart. Spray that down right toward the center of the bearing/tub area as near the back as you can get it in there. I always (mine has two holes) spray both of those vent holes in that area while guiding the tube down on the bearing. If you have four, then spray down toward the center through all four.
I have done that several times on a twelve year old dryer and that solves the problem for about six months, and then I give it another shot. Just point that little lithium spray tube in there and give it a few shots. Don't put too much in there, but spray toward the bearings in any hole that is around that bearing and it will do the job if that is your problem. This is about a five minute job if you can get to the back of the dryer easily. Try this fix first and remember, it will take a few minutes of running to spread that lithium in there and do the job for you.
If it is the bearing in back of the dryer tub that needs replaced instead of lubricated, then you have to take the screws off on front of the dryer and the tub will slip out of that bearing and you can replace the bearings. Don't forget to remove the belt off the pully first if you do that. I would suggest you have someone do that for you (repairman) unless you are very mechanically inclined.
Note: I have a friend who is an appliance repair person and he told me about both issues and the first time it was the idler pully, and the second time I had the issue, it was the lack of grease on a metal to metal bearing. Worked like a charm both times. Good luck and let us know how it comes out.