I did a google on GD-87 shutter sound and I found a page that says you can but you have to upgrade the firmware with unsupported firmware. I guess its a privacy thing so the shutter sound is always on for...you know
http://www.gd87.it/eng/faq.html
Is it possible to eliminate the shutter sound?
Yes, it is, but only by upgrading the firmware with unofficial firmware versions.
unfortunately I dont know where the unsupported firmware is. Maybe you can dig through google and find it.