What type of knocking is it? I mean, is it a metallic high pitched sound? Or is it more of a deeper thumping sound? If it goes away when you disengage the clutch, it's probably something driveline related. I know that axles can make a thumping sound upon acceleration if the inner CV joint gets damaged, that may be a possibility. No way to know for sure without hearing the sound. It could just be something loose that rattles.

As for the valve adjustment, doing one can't hurt, and it is possible that a valve could be rattling a little. But, if it goes away when you disengage the clutch, I doubt it's a loose valve making the noise. Good luck, let us know what you find.