If you don't have a tension gauge you can make a general assessment by twisting the belt. You should be able to twist it easily to a 90 degree angle, then it should stop twisting easily.
Mine was making lots of noise recently. The belt was out of alignment (running to the far outside) and was rubbing on the belt cover. Adjusted the wheel to center the belt. Also, having been on some dirt roads, sprayed some cleaner on it. It is much better now!