Prosport Problem

[josh3rd]

Ok members, I have a .177cal Prosport. I hear something moving/sliding inside the shroud area. The area I’m talking about is like a 4-5 inch area from the breach to where the metal tapers down to the barrel/shroud sleeve. So when I tip the rifle forward and backwards out in front of me with the butt in my right hand and the tip of the barrel in my left and dip the barrel down and up (we'll call it shaking for time saving/writing) is when I hear something is rolling/sliding.
My Prosport is tuned by none other than our Prosport doctor Nitrocrushr, who did an awesome job. The spring guide and top hat were done by a man who needs no introduction, Motorhead. Nitrocrushr helped me by trying to eliminate possible problems one at a time. First I made sure that all screws were tight/secured, I removed the scope to eliminate that possibility and still heard the noise. I then removed the action from the stock and checked the cocking lever's 2 screws (1 attaches to comp tube and the other attaches the cocking lever to the action/bottom of rifle) and they were very secure. I shook it again and I still hear it. I removed the guts and checked the spring, the guide the top hat, piston seal, the transfer port, the compression tube (which I left inside and attached to the cocking lever) and nothing was broken, damaged and/or missing. But I still hear the noise. I opened up the cocking lever all the way and left the breach area open, shook it and still heard it. I thought at first that it might be the delrin washers in the piston but when I removed the piston, those washers did not move what so ever. I am wondering if it could be a fragment of a pellet possibly. If so, then how could that be? Then I looked into the barrel from the muzzle end. Wondering if a baffle or an O-ring got clipped. I shined a flashlight in both ends and could see the o-rings and no damage was noticed nor did I see any other debris or damage.
I am at a loss. She still shoots superb and I know you guys/gals will probably suggest to leave it alone but the sound bothers me. I don't know what it is or how to find out. I do not want to remove the shroud because I am afraid of damaging the finish or being unable to put it back. Air Arms, I believe, has a lifetime warranty through PA (I think) but if I send it to them, they may just want to replace the rifle. Now if that happens, then I got to replace the JM spring with the oem one, replace the Motorhead guide and top hat with the oem ones along with the comp tube that came with it. I will also be giving up a Nitrocrushr lapped and polished barrel along with his tuning of the trigger. So I am at odds of going the PA route. So any and all help would be greatly appreciated as to my situation.

[Shed tuner]

Loose baffle in the end of the shroud ? Or a piece of pellet / crap in there (the last 4 inches at the muzzle).

Can 't think of any other enclosed space that could be capable of retaining an object whilst allowing it to roll around...

removing the shroud requires moderate distributed heat, and some welders gloves. Worst case even if you do damage the finish, it's £20 for a pro reblue on that tube. But shake rattle and roll with the muzzle down first - it should just fall out.