One of my ferrets sounds like hes congested?

I have two male ferrets around the age of 6 mos going on 7 mos. Only one of my ferrets is making a sound as if hes congested, Im worried he might have the cold or the flu. As far as I know this has been going on for at least a couple days, hes still very playful, has an appetite, is still using the box normally and poo looks normal. What could be wrong with my fuzzie?