WebRTC recording audio has buzz

I'm using Release 34 (Safari 11.0, WebKit 12604.1.27.0.1)

I use the following code to get microphone input and forward the audio to speakers -- note that I use headset so no feedback.

var _streamSource;
var _audioContext= new webkitAudioContext();

function startMic() {
     var constraints = { audio:true, video:false };
     navigator.mediaDevices.getUserMedia(constraints).
          then(micStarted).catch(micDenied);
}

function micStarted(stream) {
     _streamSource= _audioContext.createMediaStreamSource(stream);
     _streamSource.connect(_audioContext.destination);
}

function micDenied(error) {
     console.log(error) 
}

The audio playback of mic input is horribly distorted with a buzzing sound which makes it hard to listen to. I tried the same code in other browsers and the microphone audio plays back fine without buzz.