Here's my two cents' worth. I also have a 6-speed, have heard a rattling noise while accelerating. Seems to go away if I just barely put pressure on the clutch pedal.
I think I've found a likely candidate. There is a pin that connects the clutch pedal lever to a clevis that most likely goes into the clutch master cylinder. (see arrow in pictures below)
When the clutch is fully released, there is no pressure on this pin. The holes in the pedal lever and clevis are slightly larger than the pin, which allows the pin to rattle.
The mechanism is set up this way so that there is zero pressure being applied from the clutch pedal to the clutch master cylinder when the clutch is released. Any residual pressure being applied could potentially cause clutch slippage, premature clutch wear, so on and so forth.
I don't think there's anything wrong with having the pin rattle, it's just annoying. That being said, I'm also trying to figure out how to get it to stop...