I would first hook up a oil pressure gauge and see exactly what it's reading if it's running low on oil the tensioner will not hold tension on the chains causing it to rattle. It will also affect the vct. If it has lower oil pressure I would remove the pan and check the pick up for any sludge or any type of blockage. If all is good pull the valve covers/timing cover. Check all the tensioner gaskets as a leak here will cause low oil pressure. If all is good I would go on to replace the oil pump.

Hope this helps if you have any ?'s PM me.