Our pogo-stick vmax is one of the most reliable sleds we've owned. However this season we were out and I noticed a weird vibration/grinding sound thats most irritating at about 50-60 km/h with partial throttle. Also If your say, going along and let go of the throttle and coast to a stop you can hear a sort-of clunking sound once every little while from the clutch area. This sled used to do about 150 km/h now does max 130 km/h. And if your wondering it has about 6000 km's on it. Anyone have an idea what may be the problem, or problems?