M400 engines are noisier than M12's when starting from cold

[Fixanything]

I have heard/read this several times, why would this be ? may be forged pistons ?

[Gadgeroonie]

they may have a central gudgeon pin so you get piston slap

or the piston / bore clearances may be larger

[F.C.]

Forged pistons normally require bigger clearance than cast, that's why you should always warm up the engine gently,