Spring guns are, by their very nature loud, much louder to the shooter because your ear is so close to the stock and the sound of the action carries very well to your eardrum. Both through your ear and through your cheek and bones.
It may just require a strip down and relube to reduce or even stop the 'Twang', but it will still be loud to the person firing it You can dampen it to some extent, but you will never fully silence it.
Regarding cleaning the barrel.
Get some baby wipes or even strip of old rag, thread a piece of string/fishing line with a loop at on end down the muzzle (of