Is the sound of the oil tank normal after the 912 engine is hot?

When the engine is running, there is continuous Oil Tank "Burping" going on as combustion gasses leak past the Piston rings and pressurize the crankcase to a few PSI and return the Oil back to the Tank.  That's how it works.

This will continue for a few seconds after the engine shuts down until the pressure equalizes.

I suppose that if you were in a quiet location, you might be able to hear this happening.

But generally, this happens on the ramp with the wind blowing, the Cowls On and the Oil tank capped off, and you are sitting in a closed cockpit with a headset on.

You must have pretty darn-good hearing!!!