I went for a run one morning and found that the belt of the treadmill was slipping quite a bit. Pulled out the manual, tried tightening the walking belt, didn't help. Put it on the back burner for a couple days (weeks). Eventually found myself bored and took off the cover, found quite a bit of black dust. The internet said that it could be the motor belt as well.
I marked the belt and roller to see where the belt was slipping, this confirmed that it was the motor belt. I called up the support center to order the part. He had me move the tension spring up to the next setting, which fixed the problem as well. This confirmed that the motor belt was failing, so I ordered the part, $40 for the a simple belt... Well cheaper than a new treadmill I guess.
You can see in the bottom picture where I drew a line with white out to see if the pulley was slipping on the roller. I also made a line on the axel from the motor onto the belt and I could see that move when the walking belt would slip.