No schit! And pronto!
My 10 year old knows more than those kinds of mechanics.

I will agree with Tylus, could very well be an exhaust leak from the manifolds rather than the piping itself.
A piston would mean that that stroker kit is already junk.

 

A Loose plug could make a sound like that.
Also, could be a fuel injector? They make clicking sounds...

 

stock cams

 

The Mustang GT uses overhead cams so there are no lifters per say like in a pushrod engine. But there is a hydraulic component that some may call a lifter that works with the cam follower. The cam followers look something like the image below. The follower is positioned under the cam lobe and on top of the valve stem and on top of the “lifter”.