Double check that your treadmill has been assembled correctly. An uneven treadmill base will cause shifting and bumping which leads to noise.
Check the instructions and make sure that all the parts are where they are supposed to be. Then, ensure that the treadmill belt is laying flat against the base.
Ask someone else to walk on the treadmill, while you observe that nothing is moving or shifting during the activity. Taking it apart and putting it together correctly, is better than giving up on it altogether.
Give your treadmill a cushion underneath. A treadmill on a wood floor, with nothing between the machine and the floor, will make more noise than one that is on carpet. You can fix this by putting a treadmill mat or carpet remnant under the base of the machine.
There are even treadmill mats that are thicker, and designed for noise reduction. A less sturdy treadmill is also more prone to noise-making because it may vibrate or wobble back and forth.
Put it on the flatest surface you can. Running on a treadmill could cause more vibration. If you walk at an incline, you can get a great workout and reduce the noise.
Check if the belt. Some treadmills have noisier belts than others. You might be out of luck if the belt has always been noisy. However, if your treadmill was quiet before, and now the belt is making noise, you can try rotating and tightening down the belt. According to Treadmillsguru.com, the belt is often the first part of the machine to wear out. The treadmill belt can also be completely replaced with a new one, or one of higher quality.
Do regular maintenance on your machine. Bearings can go bad on a treadmill. These can make a scraping or screeching sound. Exploring with a screwdriver and your ear up close to the end of the tool, will give you an idea if a bearing is defective, and where it needs to be replaced.
Give your treadmill a thorough cleaning to remove dust and sweat buildup -- see if this makes a difference. Additionally, check all bolts to see if they are tight. Also, make sure no part is rubbing against the treadmill base or deck. When all visible parts seem to be in order, suspect that the motor is going bad. The good news is that all treadmill parts are easily replaced, when you find the source of the problem.