Gun makes noise when filling the air cylinder

Hi all,

I did a quick search but didn't come up with anything that really seemed to answer my question.

I have an Air Arms S510 Ultimate Sporter FAC (the rifle with the looong air cylinder). Love it by the way.....

This doesn't appear to be causing any issue because I've had the gun for a couple years and it's done it the whole time.

When I fill the cylinder from my air tank the gun makes a high pitched squealing noise. This happens if I try to fill it very slowly, medium speed, or very quickly.

I have a "Great White" and a "Tiger Shark" tank with all the bells and whistles.

I occasionally hear other guns at the range making the same noise when filling, but mine does it every single time.

Is there a mechanical problem I should address or just chalk it up as a quirk of this particular gun?

Thanks!