FM synthesis does a great job relative to its hardware requirements at reproducing melodic instruments but isn't especially useful for harsh percussion noises like drums. Yamaha provides separate drum noises for this purpose. The programmer has to forfeit three channels of regular FM synthesis, freeing up six operators for five rhythm instruments.
A global random level generator is implemented for this mode; it's a standard LFSR just like the noise generator on simple chips like the AY-3-8910 or SN76489, so it provides a one-bit semi-random output.[https://www.reddit.com/r/EmuDev/comments/gdhjt0/a_primer_on_fm_synthesis/ Source]
The OPL can then generate: