After more than 30 mins use a burning smell occurs. I thought it might just be the friction on the running belt as that gets hot (I use silicone lubricant on the gap between the running belt and the board). I took the motor casing off after one run and there was a distinct crackling sound coming from it, like how oil spits in a frying pan, plus it was burning to the touch.
Should also mention that the running belt has taken to slipping when using it at high speeds, whether I lubricate it or not. There's no slack in the running belt or the belt that connects it to the motor.