I found this today while nosing around the web. I think it would be useful for comparing prospective compressors against other recognizable sounds. I notice that not all compressor manufacturers will state the noise level of their product. I hope this is of some value to you.
Noise Levels
Noise levels are measured in decibels (dB). The higher the decibel level, the louder the noise.
Painful: 140 dB = air raid siren, jet engine
Extremely loud: 110 dB = rock music, model airplane
Very loud: 80 dB = alarm clock, busy street
Moderate: 50 dB = moderate rainfall
Faint: 30 dB = quiet library
Larry