[SPARCbook] buzzing noise from ac power supply

[nm]

I recently bought something called a "power jump".  This is
a little box that plugs into a car's cigarette lighter and has
has one 110v AC outlet on it (like in a house).  Anyway I can
plug the ac power supply that came with my sparcbook 3gs into
this 'magic box' and everything works as expected.  The only
thing that is strange is that the ac power supply (the thing
that came with the sb3) makes an audible buzz or hum noise.
It does not make this noise when plugged into an outlet in a
house.  Is the ac powersupply being damaged?  Will this shorten
its life?  Any ideas or similar experiences?

thanks,
Nick