This is going to sound like it's coming from left field, but are you sure you know which end the sound came out of? I've been surprised/confused/disgusted/impressed by a few noises coming out of my own snakes only to realize that I had a fun mess to clean up shortly after.

If in fact that it wasn't a "digestive noise", just keep an eye/ear out for him. If it continues to happen on a regular basis, then it would be worth looking into. For now, I'd probably just chalk it up to "BPs are weird" and not get too stressed out over it.