My 400 does the same thing towards the end of my shot string. It's the regulator functioning. Some of them squeak, some don't. Some squeak the whole string, some (like mine) only start to squeak at the end of the string.
If it really bugs the heck out of you, you could install another reg and hope it doesn't do the same thing.
You could also try taking your reg out and then put it back in again. it might help alleviate the sound. Possibly add a light coat of silicon grease to the outside. You could also rebuild it but, there's no guarantee that you'd fix the sound and you could screw up your reg and/or your setting if you don't know what your doing.
Personally, I like it. It's a reminder for me that I'm getting close to refill time.
Tom in Colorado