twin force problem with transmission

Hi Folks,

Im going a bit mental ..
had my twin force for 2 years and always sorted every problem..
running it brushless and its a beast !!
Uprated diffs/drive shafts and axle housings.
Since ive had some replacement electrics and made a new motor mount..
it clicks and catches when i accelerate...sounds like diff teeth slipping but its not ive checked them...with both axles disconnected it seems ok..as soon as load is applied the clicking comes back...i can even hear it when i push the truck along....could it be the chain ? i stripped the gear that it goes round after i tightened the slipper more to make the clicking worse so i could find what it was...seems strange tho...the chain seems loose but is the correct length...you can see it going round when i push the truck but it seems not to slip even though i can hear the clicking...sorry to waffle on but i have no idea what to do !