Just try to watch it with a surround setup and you'll see (doesn't matter if it's PC or XBMC). There's some funny noise in the surround and center channels while watching the whole episode 4.
It's not that big deal but it's a little pain in my ears to follow the dialogues. It's like botch stereo channels does have a little different timing.
All the previous episodes are fine.
Check the inside of your speakers for bees, houseflies, damselflies, moths, etc. If you're in the northern hemisphere, it just turned spring, so the season for these things just started. Sometimes the speaker making noise at certain frequencies can wake up the creature, causing it to flap around and make more noise / distortion.
If you still hear the funny noise, but you don't see anything inside or outside your speakers, it must be something invisible like a fairy. In this case, there's not much you can do.
If this is a constant problem, consider getting a pet spider, a carnivorous plant, or some other plant/animal that eats bugs.