It's not just you handsy - I get the same. Not all channels though. HDMI override is on but noticeably on BBCHD and ESPNHD...the same popping. I think BBC were aware of the 5.1 problem but I'm not sure if it's been fixed or it affects everyone.
For the record, I have tried the optical out on BBCHD from my FreesatHD box and still the same. So it's either BBCHD or my amp