Both Hershey and Nestle' shake occasionally when I take them out to play.
I've been told (and my own experience seems to agree) that it indicates
they need more exercise.  Mine only do it when I've had them in their cage
for too long, such as while visiting with them.  It goes away after I get
back home and they get their regular few hours of exercise for a day
or two.
 
E.S., Hershey and Nestle'
 
[Posted in FML issue 0610]