We use an engine steth (how people started calling us treadmill doctors) to find a specific bearing that has gone bad in a treadmill but don't worry, you do not have to run out and buy one. If you don't have access to one, you can use a long handled screwdriver and touch it to a non-moving portion of the rollers and motor and then put your ear up to the handle and the screwdriver will amplify the noise of the part so you can find out which part is making the noise. If it is a bearing, it will be obvious which part is the problem because the grinding noise of the bad bearing with be amplified by the screwdriver or engine steth.