My PN63B590 also has the two click sound when I turn it off and on. It's probably the switch or relay closing and opening. I'm pretty sure that this is normal, if you have no other problems I really wouldn't worry about it.
I recently purchased a PN50B650 and noticed that when it turns on it give off "two" faily loud clicks, same for when i turn it off (another two fairly loud clicks). It this considered normal operation?