(
SynthDef(\grainFMPad, {
    |out=0, bufnum=0, rate=1, centerFreq=200, dur=6, amp=0.3, pan=0|
    var trigRate = 20; // how many grains per second
    var grainDur = 0.1; // duration of each grain
    var snd;

    snd = GrainFM.ar(
        numChannels: 2,
        trigger: Impulse.kr(trigRate),
        dur: grainDur,
        carfreq: LFNoise1.kr(0.2).range(centerFreq * 0.5, centerFreq * 1.5),
        modfreq: LFNoise1.kr(0.1).range(100, 300),
        index: LFNoise1.kr(0.05).range(1, 10),
        pan: LFNoise1.kr(0.5),
        envbufnum: -1,
        sndbuf: bufnum,
        rate: rate
    );

    // Smoothing the pad
    snd = snd * EnvGen.kr(Env.linen(2, dur-4, 2), doneAction: 2) * amp;
    Out.ar(out, snd);
}).add;
)

(
// play it with your buffer b
x = Synth(\grainFMPad, [
    \bufnum, b,
    \centerFreq, 250,
    \dur, 12,
    \amp, 0.3
]);
)