It might just be the speakers turning on and off...it happens when I preview a ringtone...the buzz comes on then stops after the preview stops. And I really only notice the buzz in quiet places. It is an annoyance, but the phone is so awesome I can't complain. I just want to get the most of it. It sounds as if this is normal though.
Oh, with headphones, this doesn't happen, so probably, again, just the speakers turning on and off. This didn't happen with my S4, but oh well.