Sciontist,
As far as your VW Fox goes, have you ever done a valve adjustment? That could fix your problem right there. Overtime, the valve clearances can change, and will make that "tapping" noise.
Now for the tC, does this "tapping" noise happen after it had been warmed up all the way? When you first start it up? Other than that, I have no idea... I don't personally own a tC at the moment, so I cannot think of what it might be, unless I hear it myself.
Btw, why can't you open your hood and hear it for yourself? Can you mess with the throttle, to hear it at different RPM's? If you can (I wouldn't know.. never seen much of the engine bay. I just know the intake manni' is in the back) maybe you can pinpoint where the sound is comming from. i.e. valve cover, heat shield, ect.
Good luck.