Probably the best thing is a piston sleeve rather than a silencer. Most of the noise will be generated by the action rather than the muzzle report.
Also it may be that someone has been lubricating the compression chamber with 3-1-One and it is dieselling like a good 'un.
I suggest a strip, degrease, relube, piston sleeve of plastic or metal and try it again on the old noisometer. In terms of silencers I am sure an old BSA type or similar can be adapted to fit, tell us your barrel size. Likely that the silencer will be worth more than the rifle though.