Your tick going away with the gauge inserted , then coming back could be caused by a cupped tappet (wear in the center). Gauge bridges the gap, Quiet. Pull it out. Noise back. Others have posted that finding here. One consolation is you probably won't burn a valve when it's ticking. loose better than tight.
EDIT: and if you find that is the problem, I have some that I can pull from a block that is headed for the scrap heap.
You can have them for the cost of shipping. PM me if you need/want some. May be a day or two delay digging out the valve spring tool, and getting to the block.