My fourtrax has a slight rattle, when I try to adjust the tensioner
it does not slide in like it should. its a used utility, runs great otherwise.
any suggestions?

 

Do you have it locked in the full back position before installing it. If you didnt a drew it in tight with the bolts it will put tremendous force on the chain and stretch it out and wear the crap out of the guides. If you ran it even for a minute you will need to replace the chain and the guides. Your oil will look silvery if the guide wore through.