Hey - brand new to these forums here - i've got a 2019 Mule SX 4x4 with 3,000hours on it.

Last Nov. i had to swap out its transmission because i cracked the transmission case beyond repair. I bot it used off ebay, and it is a transmission for an earlier model, however i made sure all the oem part numbers were an exact match. The swap went smoothly and i was up and running in no time. The ONE THING that seemed odd to me, and new since doing the swap, was a strange fluttering or popping sound (more of click click or fluttering) every once and a while. Its hard to truly pinpoint the source, but i suspect its related to the propeller shaft somehow. It only happens once and a while, but most consistently when i'm traveling downhill -its in that brief "sweet spot" where its not gear breaking and its not free wheeling yet... but i've also heard the sound when i'm reversing and slightly turning as well. I wish i could be more helpful as to when it happens...

Reason i suspect the prop shaft somehow is that on my machine, even before i made the transmission swap, i noticed the propeller shaft appears to move in/out consistently about 1/2" when i'm driving - i can't see it happen, but i can see about 1/2" section where the steel is nice and clean without any grease, like its being cleaned off by movement. Its this section beyond the first 2 U-joints from bevel gearcase, where it passes into the frame & directly on the other side of the frame is the carrier bearing. Hopefully that makes sense to people. Just before this "cleaned" spot there is a flimsy metal washer that doesn't sit tight against anything in particular, though it seems it should sit snug against the frame where the prop shaft passes through. There is a lot of loose grease that's built up there, and i imagine that washer might keep the grease where it should be if it was snug, but there isn't any mechanism to hold it snug at all.

Considering this scenario was present prior to the transmission swap, i've been hoping it was normal from the factory, and that the prop shaft is designed to move forward and backward slightly with the machine's movement or flexing. If not, then perhaps its gradually worsened and the culprit of this noise?

The svc. manual's instructions for removing the prop shaft were simple and straight forward, and it def. involved pushing the prop shaft forward towards the carrier bearing (once it's bolts were loosened), but i wonder if i've missed something in the re-instlall. I did make sure the carrier bearing/housing bolts are all snug and in place, and the u-joints are all sound.

In the event what i've described is either normal, or unrelated to the strange sound, could it be related to my bevel gears? Everything has always shifted smoothly for me. The sound only starts up once the thing is in motion.... and again, its not all the time. Fairly inconsistent i guess.

I've attached an image from my service manual that represents the section i'm referring to. Oddly, neither the manual or image refer to this out of place or loose washer. Hopefully the image will clarify things a bit.