So we were riding last weekend and my 800 trail suddenly started having issues when you let off the gas.  Riding on level ground (in 2wd about 12 mph) as I slowed to enter a shallow mud hole, the tranny started making a sound like you were grinding the gears to put it in gear while moving.  I hadn't shifted speeds or even went to 4wd at all for some time before, the noise just stared all at once.  I did what most would do, stop, looked under the unit for sticks or leaks, tried high gear, hit reverse and 4wd but the same sound happened whenever moving forward. I noticed I could keep my RPM's up though and ride the brake to minimize the chatter and vibration it caused.  After I got the unit home I pulled the CVT cover, belt and both pulleys just to take a look and found all to be in good shape, just a little dust as expected for a unit with only 800 miles on it.  With the pulleys off, I shifted the unit (while off) thru the gears to feel for slack and all drive gears engaged as they should, except for the Park position, the gearshift main shaft spun freely in both directions whenever the unit was in park (and neutral as expected).  Any help or suggestions are appreciated, or if this is a repeat post, please point me to the thread that may have discussed this before. 

[2015 Z Force 800 53/Trail EPS]

One of the things that can happen is that as these engines are rubber-mounted, they have a slight movement between applying power and decelerating. Slight but it's enough to move things that are fixed to the chassis to the engine like the exhaust, which has spring joints...and the shifter and its linkage. If that linkage is out of adjustment enough, that movement can be enough to take it out of the detent enough to "grind-gears". It doesn't take much of this to permanently round-off the dogs and receivers. May be too late but...maybe not. First thing I would do is inspect the engine mounts to make sure the bolts are tight and no sign of the rubbers coming out. Next I would loosen the locking nut on the bottom of the shifter handle and back it way off. Then screw the handle down as far as it will go..another couple of turns. This will force the shifter detent lock down and stop it from locking the shifter. Now only the transmission's detents will indicate what position it's in and the shifter handle will now move with the movements of the engine...and it IS NOT locked in place so don't bump it when driving. Take it for a run. If it stops grinding gears just adjust the linkage so that the two detents coincide or are in sync. If it does not, well there is damage in the transmission such as a bent shifter fork and now some other parts.     

Thanks for the reply, it took me a bit to get back to the issue.  I tightened down the the shifter knob and pulled the unit out to test and the chatter started immediately 20 ft. out of the garage, so I stopped to fidget with the gears.  When I tried to pull back out the rear end was locked up completely. I was finally able to lock the unit into 4wd and back the unit into the garage for another inspection.  It turned out that the noise was further back than I suspected, the bolt holding the pinion in place had snapped and the pinion bearing was shot causing the pinion to slip on the gear. Luckily no teeth were broken and it looks like I can fix the issue with some bearings and seals. 

[2015 Z Force 800 53/Trail EPS]

At least it's not the transmission. That would mean a full tear-down.