Product review (5 stars): Solved my rumbling noise problem
If your dryer is making some loud rumbling noises, especially when it first starts, this might be the culprit, but you should first take the front off and do some troubleshooting. Basically, unplug the dryer, take the front off with two screws near the bottom, and manually turn the drum. Listen for the noise. Reach back and release the tension from the belt tensioner and turn the drum again while the belt remains stationary.  If the noise stops, it's likely this blower fan. In my case, the flat portion of the center hole in the blower fan was worn where the hole had become perfectly round, which resulted in the fan spinning freely on the motor shaft.  There are some very helpful videos on YouTube that explain how to replace this fan. You shouldn't have to move the dryer from its current location for any common service, including this one.  The only special tool required are snap ring pliers.