I have a 2515R with a knocking noise that was there from day one. However, I did not discover it until I had the tractor home and put it to work with a brush hog mower. Then I needed to up the rpm to PTO range and at around 1700 RPM the noise was very noticeable. I plan to address it at some point, but my dealer was a satellite of company in Oregon and folded up shop a year ago - so no help there. I did find this uTube with the identical noise on a 3520H that the owner was able to resolve. He got permission from his dealer (still under warranty) to pull the valve cover and check/adjust valve clearance. He found one valve cap loose in the head! He fixed that and adjusted the valves - noise is gone. Nice video:
I plan on pulling the valve cover when the weather is warmer and the tractor is idle. Yes, I still have a year+ of warranty left, but no dealer nearby, so I'll fix it myself. If that fails to fix it then I'll check timing. Hope this helps.