Your Quest isn't breaking the sound barrier unless you're shooting lightweight alloy ammo. If that's the case, shoot some heavier ammo and it will quiet down and gain accuracy.
The nitro piston guns might be a little quieter, but they're still fairly loud. Springers are loud, there's no real way around it.
Someone mentioned the Benjamin Marauder. That truly is a fine recommendation. You probably can't come close to getting anything that quiet for the money. If I were serious about air rifles, that is definitely the rifle I would pick. PCP's are at least slightly quieter than springers by nature, and the Marauder has a shrouded barrel that makes a difference. The Benjamin Discovery, for example, is a similar rifle but without a shrouded barrel and louder. Also, I have read that the .25 caliber Marauder is louder than a .22, and with a .177 you risk shooting supersonic. I would stick with the .22 caliber.
If you want to stick with a low budget, multi-pumps are generally quieter and also cheaper than springers. A Daisy 880 accounted for literally thousands of starlings, pigeons, and European sparrows through my youth. It had plenty of power for those. I have also witnessed a squirrel dispatched cleanly from approximately 15 yards with a Crosman 1377 pistol and a standard hollow point pellet through the lungs. Shot placement should be critical with any airgun anyway...
