06 650 h1 pieces of nylon or plastic in oil drain inspection cover

Hello I’m new to the forum so excuse me if i step on anyone’s post. I recently purchased an 06 650 h1 with a knock sound while engine is idling. I brought it home and the first thing I did was drain the oil. It was horribly black and really thick. I then took off the oil drain over to check out the screen. The screen was pretty clean but the cover had remnants of either plastic pieces or nylon pieces laying in the sludge at the bottom of the cover. I plan on putting new filter and putting in new Rotella t6 oil and seeing if the noise goes away, unless anyone has an input as to what else to look for before running the engine again. If I need to tear into the engine, I am more than capable of doing so. Also one more question is where to find a decent service/repair manual for my atv. Thanks so much!