Pan modulation

SynthDef(\sin, {
    arg freq=440, atk=0.01, rel=1, amp=1, pan=0, modAmp=1;
    var sig, env, mod;
    sig = SinOsc.ar(freq);
    env = EnvGen.kr(Env.perc(atk, rel), doneAction:2);

    // Modulation signal
    mod = SinOsc.kr(freq:20, mul:modAmp);
    // Modulate the signal between L & R speakers
    pan = pan * mod;

    sig = sig * env * amp;
    sig = Pan2.ar(sig, pan);
    Out.ar(0, sig);
}).add;