Have the boot_mute="YES" option in /boot/loader.conf since release 7.0 Now with 9.0 and 9.1-beta1 when I plug in a USB thumb drive I get no kern messages like normaly came out before in 7.X & 8.X releases. If I comment out the boot_mute="YES" option then I get the normal kern messages again when plugging in and removing USB devices. Since this problem does not occure in 7.x and 8.x releases would seem that something changed in the boot_mute="YES" option for the 9.x releases.
I see in release 10.2 that the boot_mute= option has been removed from [man loader.conf]. So I guess this is the way this problem was fixed.