Not to dig up this semi-old thread, but the .22 rimfire subsonic loads are the way to go.
I have 3 high end air rifles, 1 in .177, 1 in .22, and the last one fires an odd 9mm pellet. The .177 is OK for pigeons and small rodents, and the .22 is good for some larger pests, but there is no comparison between a lightweight pellet traveling 800FPS and a projectile 3 times heavier traveling at 650-700FPS.
The air guns are actually suprisingly loud, louder than the report of the .22 Rimfire subsonics. The 9mm is even worse than a supersonic rimfire round. Plus with most high powered air rifles, quick follow-up shots are measured in 10's of seconds, as opposed to your typical bolt action .22 rimfire. Again, the 9mm is the exception because it uses a CO2 bottle and is lever action.
The higher end .177's from Beeman are nice air guns. I used to kill pigeons with 8.5 grain hollow points out past 80 yards from my back porch with no trouble (If you've ever tried to kill a pigeon with an air rifle, you'd appreciate how much of a feat this really is
)
Hope that helps.
Jason