Quoted from michi:
If this happens when not in "Collect Crew" mode, it's most likely a chattering opto on the chest (which makes a similar sound for getting each key letter). The contacts for the optos where they plug into the board are eternally unreliable. Usually, pulling the opto plugs off the board and re-seating them fixes the problem, at least for a while, so I'd try doing that first. Also re-seat the main connector (8 pins, from memory) for the opto board; it can cause this issue as well.
I'm still looking for a good solution to permanently fix the problem with the poor circuit board connectors for the opto boards. (It happens even with brand-new board and optos, as I found out the hard way.) If anyone has any suggestions, I'd be keen to hear them!
The re-seating is working thus far and hopefully for good. Thanks for the tip.