I'm aware most plasmas have some degree of buzz and that it tend to vary with light output. A minor amount (similar to hum from a fridge) I have no problem with. This was indeed loud, it's a shame I didn't walk away from the TV and continue with the video. You could hear it anywhere in my loft, which isn't the quietest place itself as I live in the middle of downtown San Diego so there is always some ambient street noise.
That said, do you notice a pattern with the noise like in the link I have? IOW, the moment the TV is turned on it gets quite loud for a second or two then settles down.
The retailer has coordinated a stop on the return shipment and to have a Panasonic tech fix the unit at the warehouse. Hopefully it's a simple fix, I got a great deal on this open box unit aside from the some cosmetic blemishes on the back and this noise it appeared to be in good shape, the uncalibrated screen out of the box was mesmerizing. If I send this back I'll probably get a new GT50.