Ticking valvetrain noise from fresh engine

Hello, spent some time on the engine but the noise is still there.

Here´s what I did, one step at a time:
-new header gaskets (old ones were fine)
-shimmed the rockers up 0.020" to reduce preload
-checkt the rocker bolts length to make sure they dont bottom out
-made a leakdown test, all cylinders are 90-95%
-checked valve height with a straight edge
-switched all rockers from driver to passenger side

I have been driving approx 150miles with this, it´s neither getting worse nor better.

Everytime I start the (cold) engine, the valvetrain is quiet, but the ticking starts after 50-60 seconds.

The engine is running great, nothing else to complain about.

WTF is this!?!

Thanks
Chris