What could the knock be

How many miles..

If it is white smoke then you probably blew a head gasket which in turn spun a rod bearing which would explain metal in the oil and the knocking noise!

Oil burns blue, white smoke indicated water in the combustion chamber - Most likely head gasket, cracked head or block. How has the coolant level? The knock you describe sounds like a lifter issue from the valve train, but the source can be deceiving when you listen with the naked ear.