11xx is an effect of multiple uses, including:

NEC PC-Engine

11xx sets if noise output from the channel it is used in. It only works in channels 4 and 5.
If XX = 00, noise will be disabled, if XX => 01 noise will be enabled
N.B. This effect will only work if 17xx is disabled.

Nintendo Game Boy

11xx sets the polynomial counter for the noise channel, if XX = a multiple of two, it will be set to 15 steps, otherwise it will be 7 steps.

See also