Mine also buzzes and it irritates me a little. I tend to be more prone to noticing those kinds of things, though. I think most people wouldn't really notice. It buzzes more loudly when bright screens are displayed.
I don't know how loud 15-20 is on the TV (I have audio directed through loudspeakers), but I think chances are that your TV is working fine and it's just something you will need to adjust to hearing. Of course, I cannot tell you for a fact that it is fine or not because I am not there experiencing it. However, it really comes down to your enjoyment and whether you can accept that buzz. If it's too much, then just know that if you return it, you may still end up with another TV that buzzes similarly.
I just wanted to offer you my opinion, because I, too, came from an older LCD that I have been using for 5-6 years. I am acclimated to how the LCD did not buzz and now I am hearing the plasma and, at times, it irritates me that I can hear it, but I'm fairly certain the set is perfectly fine.
Thanks for the feedback guys. I ended up swapping it out for another and unfortunately it's buzzing just as bad. I can hear it over the sound of the TV when it is up above reasonable listening levels, at 30+ on the volume bar. At this point I'm at a loss for what to do. I'll probably hang on to it for as long as the return policy will allow, in the off chance that my ears adjust and I simply tune it out. In the meantime, I'll ponder switching back to LCD or hope by some miracle I stumble upon a Panasonic S60, which seems highly unlikely. Thanks again for your help!