This is just for information purposes in case someone ever has a problem like this.
So, I parked my red bike about two and a half years ago after it developed a tap that got consistently louder with additional miles. This occurred after I blew one motor then got another of unknown origin with (according to the odometer) less than 50k on it. It had a slight tap when it started that wouldn't go away (even after a few valve adjustment checks. Over a few months it got worse so it got parked until I could diagnose it. At least I had another one to ride. I spoke to SISF about it at the time and he had a suggestion on what it may be, and as I suspected, it involved removing and inspecting parts in the top end. The thought was that rockers can wear down and start to tap when their seat is deformed. So, I reopened it last week. Both cams had great looking lobes so there was no real tell-tale sign of where to start looking. I started with the intake cam. When I pulled it you could feel the problem child with your finger pretty easily. All of the rockers when new have a seat that is arched upwards, kind of like a roof. The rocker on cylinder 4 was dished inward (or downward). The picture below illustrates this. The worn one has a tiny bit of pitting on the edges but no galling. It also has slight lip on one side. I'm guessing it just wasn't case-hardened properly. The oil port seems to be the same size as the others. I took one off my parts engine and replaced it. I inspected the exhaust cam also and found nothing. I was nervous when I restarted it. However, it was quiet immediately. What a relief!
Hopefully, this may help someone in the future.
Thanks again Steve! You see, we listen to you sometimes. Now I just have to fix that leaking balancer shaft seal.