Blackberry Passport buzzing during calls?

Hey everyone, just picked up a blackberry passport and I love it. I currently live in Canada (GTA) and am with Rogers. Sometimes, although not always, when I put me ear to the earpiece and engage in a call, there is a slight buzzing, like there is a subtle rumbling in the ear speaker when someone is talking to me through the phone. It increases when I turn it up, and decreases when I turn it down. It's important to note that this doesn't happen all the time, i.e., a phone call started like that (with the buzz) and I moved around in my house thinking it was reception, turned it up all the way, and then no problems whatsoever. Any ideas?

Thanks!

Nearby wireless electronics? Microwave?

Blackberry Passport running 10.3.2.798