Is it a douple knock? example (knock-knock....knock-knock...) or a single (knock...knock... knock...)
single knock is the main bearings on the crankshaft, one knock per rotation, douple knock is the main bearings on the pistons, one knock at the top of the cylender, and one know at the bottom. or it could be both. if you have warrenty id bring it to them, its a big job.