I now use exactly one 38mm thick fan in my daily driver as the pull fan in a push-pull setup.
It was just unbearably loud with 12V though, so I modded the molex connector to connect to the 5V power wire instead.
At 5100-5200RPM with 12 volts it was like sitting in a room where someone has either a vacuum cleaner, a hair dryer, or a small jet engine on full blast right next to you. Now at 5 volts she's spinning at 2300-2400RPM and is almost unnoticeable, there is a slight bit of bearing whine though, but it's quiet other than that.
I don't think I should be able to hear my PC from the other end of the house while I'm watching TV.