Disable serial hint if needed
Comment out uart hints in file /boot/device.hints
#hint.uart.0.at="isa"
#hint.uart.0.port="0x3F8"
#hint.uart.0.flags="0x10"
#hint.uart.0.irq="4"
#hint.uart.1.at="isa"
#hint.uart.1.port="0x2F8"
#hint.uart.1.irq="3"
Set UEFI loader console to null
/boot/loader.conf
console="nullconsole"