Have a look at these links ->
The GBA's noise channel is the same as the GB/GBC's, just so you know. Essentially, the output frequency of the noise channel is
The dividing ratio is 0.5 for a 0, and 1.0, 2.0, 3.0 and so on for values of 1, 2, 3... The shift clock is basically 2 << (top 4 bits of NR42). This is the frequency you run the LSFR (i.e. times per second). The LSFR is basically a series XORing the first two values of a number (the 7 or 15 stages, that is to say, a variable holding a 7-bit or 15-bit value), shifting that number once to the right, then replacing the most significant bit of that number with the results of the XOR. The output of the noise channel itself is either ON or OFF, high or low. The ON state is determined if the least significant bit of 7-bit or 15-bit variable is 1. Otherwise, if the least significant bit is 0, the state is OFF.
(524288.0 / DIVIDING_RATIO) / SHIFT_CLOCK