Do you have another source (like a DVD player) connected to the set, and if so, does it have this problem also?
Did this just show up one day all of a sudden?
Call a Sony authorized service center and tell them you suspect your optical block is defective. Try to have your original bill of sale handy, and they should take of it for you.
Hope that helps, let me know if you have any other questions, and thanks for using JustAnswer!