Under the valve cover - Prius edition

Beautiful shot, nice to see something so well maintained.

Just my .02: forget about that timing chain. You just happened to remove the valve cover while the chain was in a slack position. When the engine is running and the tensioner is supplied with oil pressure there's no slack to be had.
I’m hearing a little chain slap under load, not quite the death rattle a Ford 4.0 SOHC Cologne V6 or a 5.4 3V Triton in an F-150/Expedition/Navigator makes when it’s time for new chains. But it makes itself known on long on ramps.

It’s not unheard of to replace a timing chain on a Toyota NZ or ZZ series engine.