Purchased the Yamaha 5740 A/V Receiver

Hi ... I purchased the Yamha HTR-5740 receiver this past weekend. I got a good deal on this unit paying under $250. Although I have not had alot of time to play around with all the settings, I am very impressed with the sound and functionality. I added a coax connection to my DVD player, both movies and CD sound terrific The only thing I have noticed is that when watching TV using the DTS Prologic mode, there is an annoying buzz coming from the center channel speaker. I do not hear it on any other sources, only when viewing HBO or other movies on TV. Note It does not occur when watching TV using other surround modes on the unit. Any initial ideas what this may be? I hope to investigate further this coming weekend. :) Thanks in advance