it could be black death, this is when your oil has bits off crap in it, what happens after a while is the oil pick up strainer in the sump gets covered in shite and then this stops oil circulation to the top end of your engine causing oil starvation to your hydraulic tappets making your car sound like "A SUBARU" to cure this i would take of your engine sump and clean all the crap out also clean the oil stainer pipe and give it a nice oil and filter change