engine knock

Hey guys I need some help I just bought a 2011 RGE rzr4 800 with only 1000 miles on it when I got it home it ran great just a little lopped idle but it has good power and ran out smooth the guy said he only had it out one time in the past 2 years and it needs serviced so I got the stuff from Polaris dealer on the way home but didn’t change it right away I just checked everything to see how it looked the oil was fine looked fresh so I took it riding a few times last week just a few miles back on the hill behind my house nothing rough the rzr just made it almost like driving down the interstate easy stuff but when I got home from the last ride I noticed the engine was tapping or ticking at idle so I figured the oil needed changed maybe it broke down over the winter but after the oil change I drove it up the road when I get back home the tap was a lot louder so I checked the easy things clutch belt anything that could move or rattle there was no change I even took the clutch off it didn’t change it kind of sounded low in the engine so I tore into into it I’ve got it down to the rods there’s no slack everything looks and feels new the only thing I can find is the lifters all but one the hydronic part on the inside are stuck or aleast so hard I couldn’t push em in at all even with a vise and a old push rod I had laying around the engine from what I’ve read an seen on YouTube looks new no ware on anything I even cut the old filter apart and it was clean
This is not my first time I’ve been turning wrenches for years a little bit of everything I’ve rebuilt almost every machine I’ve had and a lot of my buddy’s stuff to this is my first rzr tho it’s easy but I dont know if the lifters would cause it to knock like that and not bend or make it run like crap the rods are all fine to