What are you shooting? Type of rifle? I have a Umarex Ruger Blackhawk break barrel in .177 . It too sounds like a 22-rim fire with light pellets. I moved up to 10.5gr lead, and that has helped. I too don't want to alarm nosey neighbors. Now trying to find a pellet/slug that has the best accuracy. By the way, why not lead?
I'm shooting with a Gamo Wildcat Whisper. Currently using their Hunter Impact County. They shoot great and very accurate. I tried a RWS Hypermax "tin" today and it is too loud.
We are surrounded by woods and even have hawks that love dining on the downed squirrels.
Just hoping to find an more environmentaly friendly alternative to lead.
 
I've had good accuracy results with an H&N "Power" copper-coated lead pellet. Not sure if coated lead meets your requirements vs. non-lead alloy.

Non-lead pellets certainly can go supersonic and that's not subtle. There is a minimum weight you'll need to stay subsonic. Pyramyd has a nice filter feature to show only non-lead pellets.
 
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