piezo pick hum

["Michael Peters" <mp@mpeters.de>]

http://www.michaelpeters.de/temp/soloette-hum_0001_0.wav

When I plug my Graduette Soloette guitar (a nylon string travel guitar with
a piezo pick) into the external soundcard of the laptop and launch Bidule 
to
hear it, I find that it hums - a deep drone hum plus a high whining sound.
This does not happen with regular electric guitars. My technician says it
has to do with the piezo. When I put a finger on the jack cable that sits 
in
the connector in the guitar, the noise stops.  

I can get rid of the deep hum with a plugin, but the high whining noise is
still there. I can't play with my finger on the jack cable all the time. Is
there a simple way to get rid of that noise?  a special VST plugin?  or 
some
kind of grounding cable that I connect to the plug? and where does the 
other
end go?  I need a quick and simple solution. Help!

-Michael