I am not suggesting this is your problem but it sure baffled me for a while. I was running a small block Ford on the track when it developed a valve train "tick." That night I was in the hotel parking lot trying to figure it out, nothing out of normal under the valve cover, everything was perfect but there was this distinct tick that was EXACTLY what a miss adjusted rocker would give.
Anyway, as you might guess, as with any work being performed on a car in a hotel filled with car guys, it generated a crowed...and everyone had an opinion. One nut suggested I had a bad header gasket, which knowing better, I wrote off, said "Thanks for the suggestion" and thought to my self "This guy is an idiot." Well, guess what, after trying everything else, I figured why not give it a try...problem solved.
Man, I still feel bad for thinking what I thought that day...I KNEW it was valve train.
Just a thought, if you can't find anything else, a gasket is cheap compared to tearing down an engine. It probably won't fix it, but, after you check everything else, why not try before tearing it apart?
Oh, by the way, the crow I ate that night was not very tasty. But is was made better when running on the track the next day.