Any piston can make noise if its clearance is too large, though 'knock' is generally not the term used to describe the noise. 'Slap' is the term.
The presence/use of forged pistons indicates a high performance application.
In short the higher output requires stronger pistons, which forged pistons are.
But with the higher output comes a higher heat load. One reason forged pistons are used is they can withstand temps at which cast pistons get soft, too soft.
But forged pistons can take the heat. But they may require larger clearance in the cylinder since they can operate at a higher temp thus they will grow more in size, expand due to the heat. Thus when cold, and maybe all the way up to operating temperature they can well, slap.
If you hear a knock that is not a noise that is usually associated with excessive (though not excessive excessive) piston clearance.
It is more often associated with a bearing (main or rod) that has too much clearance and the necessary hydrodynamic oil film that forms in the bearing space doesn't form or form fully.
(Yet another knocking noise source can be a piston pin with too sloppy a fit. Since the engine has forged pistons it has obviously been apart so any fit/clearance is suspect.)
Anyhow a main or rod bearing knock is usually not present when the engine is cold -- with maybe a brief exception upon engine start because the oil in the bearing interfaces has had time to flow away -- because the thicker oil helps to create the necessary oil film. Only when the engine gets up to temp and the oil thins out does the bearing knock appear.
I have not followed the newer Cayman engines that closely (since I lost my 08 Cayman S to an accident a couple of years ago) but IIRC there were reports that some of these engines had a marginal oil supply to at least one main (or rod) bearing.
I would not get too worked up on what I'm saying above about the marginal oil/pressure supply since it is little better than hearsay but you might want to more clearly identify from where the noise is coming from. If you can do this then where the noise is coming from helps to identify its possible cause and from this you will have more info to decide what (if anything) to do next.
Sincerely,
Rockster.
