Thanks for the replies so far.
It sounded like he was making the sounds on purpose, not like it was a by-product of a breathing problem. His breathing doesn’t sound labored or anything. Nevertheless, I’m certainly no expert. I’ve given the little guy a closer look.
No dried red stuff around the eyes or nose. He certainly hasn’t lost his appetite and seems as energetic as ever (of course, I only have three weeks of his behavior to go by).
We’ve used shredded newspaper since the beginning, so that hopefully is not an issue.
I have noticed occasional sneezing from each of them but it hasn’t seemed excessive. They’re both little balls of energy and don’t seem to have any health issues that are obvious to me.
I’ll keep watch on him though and if it persists, I’ll take him to the vet.