Hey all,
I don't believe that the chirping sound has much to do with anything.  The
chirp is made by the chrome tailpipes.  Remove them, and it sounds like a
bus.  Put 'em back in, you get the chirp.  If it's not chirping, get new
tailpipes :) .  Why VW chose to have the bugs make the chirping sound I
don't know, but I like it -- gives the cars some personality.

Take care,
Sean