I don't think its the lock. I think its probably the stop pin (the pin that the back of the blade makes contact with when the blade is open). That pin spins. Put your finger on the stop pin with the knife closed and shake it. Does the rattling stop? This is normal, and beneficial. The pin spins to reduce wear. This way, the blade does not contact the stop pin in the same place every time.

On some Benchmades, the stop pin is fixed in place, and makes no noise. On some others, it spins. Again, normal.

Or, I could be wrong.

Hey. First post!