Strange vent, sometimes damp, wet, dripy

[Clarence Brown]

Funny vent in basement. 3 in Dia plastic w/foil ring. Sometimes wet or dripy. Sometimes dry. What is it? Built 1999. Any ideas?

 

[Smooky]

Someone may have made the opening smaller by covering part of the opening with foil. If the duct work is not insulated or the foil is not insulated warm moist air will condensate on the cold surface.