Air flow switching?

I'm not sure where to start, so please bear with me. I think something may be starting to give me trouble with the air circulation in my car. Since it's wintertime, I'll start off running my car with air flow going to the floor vents. Once warmed up, I'll switch to the vents that can hit my face. I'm beginning to see that from time to time, the air flow may be weaker than normal coming through these vents and some type of humming type sound can be heard, not loud, but can be heard. I'm thinking that it may not be switching over properly and that some of the air flow is probably still going to the floor vents at this time, but I'm not sure. So, I'll typically switch back to floor vents, then switch back to face vents and it'll typically sound normal and blow normal. Could anyone point me in the right direction please?