If you use a treadmill regularly for several years, there's a chance that the walking belt has worn out. A worn out treadmill belt will cause the treadmill motor to work harder. If you don't do something about the belt, you could damage your treadmill beyond simple repair. Before you look into replacing your treadmill belt, you first need to find out if it is worn out enough to warrant the change. If it is, you can hire a repairman to replace the belt or replace it yourself.
Examine the edges of the treadmill walking belt. Worn out belts will start to curl and fray on the edges. A small amount of curling is acceptable, but if it is noticeable and highly curled and frayed, it may need replacement.
Run the walking belt slowly at one mile per hour without standing or walking on the belt. Look at the edges as well as the middle to find any rips. Pay special attention to the seam, as this is often the first place a treadmill belt will rip.
Evaluate the treadmill's recent performance. A worn walking belt will cause the motor to work harder, making it sound louder. If the treadmill works just as well as it did when you first bought it, there likely isn't a problem with the belt.
Lift up the edge of the walking belt along the middle of the deck. It should lift up about 3 to 4 inches. If it lifts up higher than this, the walking belt is loose.
Tighten the walking belt by turning the screws at the end of the treadmill deck with an Allen wrench. Turn the right and left screws clockwise the same amount for each screw. With every quarter turn, check to see if the edge reaches 3 to 4 inches in height. If it never seems to tighten, your treadmill belt may be worn out and too loose, needing replacement.