Of the 18 models examined in Sun's Entry Rackmount Servers category, all but two listed an acoustic rating for L-pAm, the noise a bystander would hear one meter away, and which is probably the best method of comparison.
The least loud was the Sun Fire X4140 (also one of the less expensive units) clocking in at 51 decibels. That's equivalent to a quiet conversation in a living room, so it at least would not be over-powering.
But the other units typically were rated in the 60-decibel range, which is the equivalent of a face-to-face conversation on the street. The Sun Fire X4500 (a higher-end 4U unit) was given a range of 65 to 75 decibels. The upper range is like living next to a busy freeway with the windows open.
The figures show the importance of having some kind of closet, enclosure, or solid-walled rack, to keep the rackmount servers from dominating the environment. Of course, given the higher price ranges represented by most rack units, the buyer should be able to justify the additional expenseor seriously consider not going that route.
Go to the bMighty Server How-To Center