Here's a possible answer as to why a sink glugs on this HGTV blog: Ed Del Grande: Ask Ed -- Glug... Glug... Glug... in the Sink
Another interesting thing about vents is the fact--as I've learned from experience--that tree frogs can go down a vent. It's really surprising when one turns up in your toilet or your shower drain. (Actually, I don't see a connection between a shower drain and a vent, but there it was. We had to take the drain cover off to get it out.) And I've seen snakes going into a vent to get the frogs. Fortunately I've never found one in any of our toilets, but I've read about them there.