or engine oil entering the combustion chamber and pre-igniting.

but yes it could be a turbo or pcv problem.

diagnosing it will not be easy.

to check for head gasket: #1 cylinder at tdc on compression stroke, remove the oil filter, put air into cylinder and then watch/listen

to check for turbo: look for play and if you have a spare put it on and see what happens. Maybe someone on here has a better idea on how to test it.