300E engine sounds like diesel upon cold start

Sorry to clutter the board fellas as I recall this might have been discussed recently but could not find the thread after numerous searches.

My 300E sounds just like a diesel on a cold start then totally disappears to a smooth whir after reaching around 50c temp. It's not a metal to metal sound, more like well...... a classic diesel engine sound -- almost like exhaust is coming out of the top of the engine.

For what it's worth, I recently had top of engine off to replace front cover U seal and valve cover gasket. I did not touch the chain, tensioner, or rails. Noise began just after re-assembly.

If someone could point me to that thread I saw or knows a possible solution to this problem please reply!

Thanks!

Iodyn