Our upstairs bathroom has a toilet, tub and sink. The sink drains very quickly, but starting a couple of seconds after it empties there is a gurgling sound (blub blub blub...) that lasts several seconds. I can even feel it when I hold the pipe below the sink. Consider these facts:
The tub does not gurgle after it drains and the toilet does not gurgle after it is flushed (I tested by shutting off the supply before flushing so the refill noise wouldn't interfere).
Two separate plumbers have snaked the sink line and declared it clear.
Someone went onto the roof to run water through the vent pipe and it appeared clear.
One of the plumbers went on the roof to snake the vent pipe. It went all the way down to where it met water when we ran the sink. There was no blockage found. When I look at the roof, I see two vent pipes, one with a narrower diameter - I think the plumber may have said that was just for the sink.
Despite the above, the gurgling persists. The plumbers have no explanation but they both told me that it's nothing to worry about.
I'd be interested in any opinions about what may be causing the noise and whether based on your similar experience it needs addressing with something not already done. Could this have anything to do with the underground sewer pipe? If so, why would only one plumbing fixture be affected?