Try a different outlet. Sometimes having your TV and treadmill plugged into the same power outlet or surge protector can cause interference to occur. Try plugging each appliance into separate power sources.
Run an extension cord in from another room. When your TV and treadmill are plugged into the same circuit, then interference often occurs. A solution to this is running an extension cord in from another room to plug in one of the two appliances. The downside to this is the inconvenience that having an extra cord running from room-to-room causes. You might consider only plug in the treadmill (via the extension cord from other room) when running, and during other times wrap the cord safely out of the way.
Switch the plug from three-pronged to two-pronged. Go to your local hardware store to purchase a converter that will make your treadmill (or TV's) three-pronged plug, two-pronged. Doing this can help change the frequency the appliance sends through the circuit and therefore may alleviate interference.
Increase the distance between your treadmill and your TV. If your treadmill is directly in front of or immediately next to your TV, ground vibrations could be causing the interference. Try moving your treadmill farther away from your TV.
Place your treadmill on a shock-absorbing mat. Many exercise equipment stores sell shock-absorbing mats. Putting your treadmill on one of these mats will decrease the amount of ground vibration that your treadmill creates and may decrease the amount of interference.