Royale Noise

I think it is the reg "recharging" the plenum? Not sure but I remember a lot of streamlines did the same thing when they first came out and mostly seemed to happen when pressure got down towards the reg set point. I'd try refilling and taking a few shots to see if the noise disappears. If everything seems normal I don't think it's an issue but someone with an FX may know the answer.

That is what I was going to say. My Wildcat makes noises when I get the pressure down to the reg setting. In fact it makes a loud bang that sounds like the moderator came off.

Crusher

2nd what tweeter said....mine too. I kinda use it (audio feedback) to give me an idea when i'm getting close to the reg setting.

I have a Royale 500 and it makes the same noise you describe. I thought it was leaking somewhere so called the dealer. As one of the first responses indicated it is the sound of air moving in preparation for the next shot.

My brother and I both own a .22 Royale 400 and both of our rifles make the noise described by the OP when a rifle pressure is low, and ESPECIALLY more noticeable when shooting a rifle during cold ambient conditions. We almost never seem to notice the "noise" during the summer months, but we hear it frequently during the colder months when shooting the rifles down to lower pressures. 

For the OP, I suggest checking to see if this noise also occurs with your rifle during low tank pressures and during colder air temperatures.

Yeah, northwest PA here. My brother and I have several off-hand, walk around trail-type courses that take an hour or more each to complete. We have more than once been out shooting when it is in single digit temps and even below zero. These cold shooting days are when we notice the "noise".