Are you guys running your supressor wet or dry? I recently picked up a sparrow and while I like it, I find the 15-22 is my loudest host. (Bolt rifle being the quietest as expected, 10/22 still quieter than the 15-22) I think it is even louder on the 15-22 than a 1911-22 all with the same Cci sv ammo. But I haven't tried it with water or wire pull gel...
Other than the first round pop, the 15-22 is very quiet. Get some aquilla 60 grain or some copper jacketed subsonic and it is really quiet. When I shoot lead balls through it is is a little louder than the good ammo.