So I have been having this issue and I am at a loss as to whether I can actually fix it or not. I'll start from the beginning.
On September 17th, I started my bike at work and headed home. When I got to an intersection, I noticed a loud clacking/knocking sound coming from my bike over my earplugs. When I got home, I took out my plugs and it was incredibly loud. So I stopped riding it.
I dropped and checked the oil - it was still within the sights and not low, and there was no noticeable metal in the oil. I attempted to check the valves, but I honestly couldn't figure out exactly how to get the feeler gauges in to read (And I couldn't get the timing cover off either) so I took it to a shop to have them adjust it. I want to note that while driving it to the shop, I noticed absolutely no change in power or response - it's just loud. The biggest thing I could see was that at least it was oily so the head was seemingly getting properly lubed. He said the exhaust valves were tight, intake were loose, he "set decompression lever cable freeplay", replaced the tappet cover o-rings, and "checked that the flywheel rotor is tight".
The sound is still happening, so I got a stethoscope and tried to listen to where the sound was coming from. I might be misunderstanding this, but I could hear the sound all the way up the cylinder. It was noticeably louder towards the bottom. When I had it in front of the starter motor, it seemed to be absolutely the loudest. But, at the same time the sound was clearer towards the top of the cylinder on the right hand side of the bike.
Here is a video of the sound, although since it was taken on my phone it is a little difficult to hear.
https://youtu.be/a3AB8_z7tooThe mechanic I had taken it to said "motor sounds like bottom end rod or crank bearing," and that it would be cheaper to just replace the engine. I am not positive that he is right though and any opinions would be amazingly well appreciated. I want to try my best to fix this.