Dripping sound in vents?

Hi all. I can't find an answer to this anywhere...which seems odd to me.

It's been very hot and humid where I'm at, and the AC has been going for quite some time (normal). I've been hearing dripping in one of the upstairs vents and I'm wondering if this is "normal" or not? Never had this happen before, sooo I don't really know what to do! Heh! Any input would be greatly appreciated!