My FX Streamline makes a different sound (dull clang) when the pressure drops below 110 bar. I assuming that this has something to do with the regulator. Is it supposed to do this?
What's your reg pressure? Guns often make a 'different' noise when coming off the reg.
Although at 110 bar it might also be that the cylinder pressure is too low to provide sufficient back pressure against the hammer and allowing the valve to bang open too far/to its limit.
Personally I'd just refill before that point so it becomes a non issue, but some of the properly technical chaps way have a more exact answer for you.