Air tank squeak when filling

I just got my Omega Trailcharger and it's been great. I am filling my 30 minute 4500 PSI bottle with it, going with the recommended duty/cooldown cycle of 30 mins on, 20 mins cooldown. I have the Omega hooked up to a Stikman, which is connected to the bottle. The Trailcharger and the Stikman both seem to be performing admirably. The only thing that gives me pause is a squeaking sound that seems to be coming from the tank neck. Every time the compressor pumps the noise squeaks. I can't figure out what that would be, or if I should be concerned. The pressure is up to about 2500 PSI. Any ideas? Should I be concerned? The tank has a normal CGA 347 valve.

Edit: Just had a thought. The valve to close is one of those "push down and turn" types, but I tightened the nut down so that it just turns like a regular valve handle now. Wonder if that setup could get some pressure on it, causing the squeak.