The term knock usually refers to ping or the sound caused by pre ignition. This is a sound not unlike shaking marbles in a tin can. It is caused by ignition being too far advanced. On most fuel enjected engines this is controlled by the ECU and knock sensor which retards the timing slightly when required. Never heard of lean mixture causing preignition though.

I think you are talking about piston slap, valve tapping or some other noise. Without hearing it it is hard to know which. If it is valves the clearance should be checked although it is rarely off spec. You do not mention mileage but sometimes we just live with the quiet noises of high mileage engines.

The cleaner will do no harm to the engine, just gets burned off.