I appologize, For some reason I assumed airfilter.

You have a oil-pressure gauge?
It might be your oilpump pressure relief-spring.

-oilpump builts up to much pressure.
-this is even worse when oil is cold (start-up)
-it will be forced out where resistence is least.
-when engine runs for a little, oil heats up and loses some of its "thickness" so oil gets pumped around easier.

Grinding noise comes more then likely from your pump too.
If this is it it's fairly cheap fix, just hope you can drop the pan with motor in vehicle.


It's a guess, but worth to look into.
Hope it helps a little.