Ticking/tapping noise, 305 Chevy, Can't Figure it Out!

[Rust-O-Matic]

Ok engine guys, I need your help. My '66 has a late 70's model 305 small block Chevy in it that is making a ticking or tapping noise from the top end, only when it's warmed up. It runs good, and sounds fine when it's first started up cold, but starts tapping as it gets up to operating temp.

I put a stethoscope to the valve covers while it was idling and the noise appears to be coming from under the front of the passenger side cover. So I pulled both covers and adjusted the rocker arms the old school way, by backing them off till they click, tightening slowly til the noise stops, then going 1/2 turn past that point. I did the passenger side 2 or 3 times, just to be sure. Noise still persists.

Then I pushed all of the rocker arms/tops of pushrods in that area around with the wooden handle from a hammer (while it was idling) to check for slop. None was noted.

Then I changed the oil from 10w30 to 10w40 to see if that helped, then added a quart of Lucas to it as well. Still exactly the same.

So now I'm not sure what else to do. My thought was maybe it's a collapsed or stuck lifter, but it seems to me that it would make noise all the time, not just when hot. I'm reluctant to tear the top end apart to replace the lifters, since I'm not sure if that's the problem, and the fact I can probably find a used 350 for not a whole lot more money or effort. Any input you guys could give would be much appreciated. I've spent hours reading different forums and links online, but haven't got it figured out yet.