What Causes Drainage And Congestion At The Adenoid Glands?

I can breath clearly through my nose but have drainage and congestion where I believe my adenoids are. It drives me crazy because I cannot blow it out I cannot swallow it, (yuk, I know). It just seems hang in that area in the top of my throat behind my nose, yet I can still breath through my nose. Every once and awhile it will build up and then I am able to snort it out and cough it up. Sorry to be gross but have to be somewhat graphic to describe what is going on. Thanks