Each pistol has a different type of noise report, and are perceived differently by different people; hence the variance in oppinions above.
We do not have the definitive noise levels (in dB), so the oppinions are somewhat subjective.
I have seen folk describe the Alecto as quiet, and others disagree. I have owned both calibres and found it similar to most other SSPs on one pump, but much louder on two or more pumps.
As mentioned the HW40 has a nice crack, at first you may perceive it as loud, then you get used to it. But if its neighbours you are concerned about, they may notice it.
The Crosman 1377 seems to produce most of the noise from the hammer spring, with a somewhat softer lower frequency sound (fewer pumps, lower velocity, lower noise report). There is a nice reliable one for sale here.