[Kestas]

How about checking the tensioner? It should be a rather easy and straightforward job. Check to see if it's reached the end of its travel, or if there is anything otherwise wrong with it.

The tensioner is designed to ratchet in one direction only to take up the majority of the slack (i.e., to "rough-in" the tension). This helps keep some tension during startup. After the slack take-up is roughed in, the tensioner further uses the engine's oil pressure to take up the remaining slack after the engine is started. I suspect the tensioner's ratcheting system may be stuck or reached the end of its travel, and the oil pressure is doing all the work. Just a theory.