2 problems with Behringer Xenyx 302USB


I have been trying to record using the 302USB (brand new) hooked up to Garageband on my Mac. I have my headphones plugged into the mixer.

Here is my current setup.

SM57 mic > LXR input on 302USB > MacBook Pro > Garageband

1st problem: I keep hearing the faintest (yet incredibly distracting) clicking noise in the left phone of my headphones. It will last for about 10 seconds, go away, then return. It's not any kind of metronome from Garageband that I forgot to turn off.

2nd problem: about every 30 seconds I hear a loud beep in my headphones. I don't think it has anything to do with signal clipping b/c neither lights that would indicate this on the mixer or Garageband turn red.

I thought this problem was related to the Garageband, but I would turn the program off and watch a video online with headphones through the mixer, and the beep would return. Could it have anything to do with sound output from the computer?