I have five 120 mm case fans a 80 mm case fan, PSU fan, 60 mm YS-tech on FOP 32, two drive bay coolers featuring four 40 mm fans blowing over hard drives, and a PCI slot cooler with two 40 mm fans and find the sound of the fans relaxing, its not something that bothers me and its not something that I have ever measured the decibels of.
A bigger concern for myself is how the fans effect sound waves and the fequency responce of my sound system and select the fans and positon them according to this as well as cooling.