Sink drain gurgles when toilet is flushed

Hi-

I have searched the forum but can't seem to find a thread that addresses this issue. I am a new homeowner, and know ZERO about plumbing, although I have picked up a bewildering amount of information in the last hour I have spent reading your posts!

Most of the time I flush my bathroom toilet (but not every single time) a loud gurgling sound comes from the bathroom sink. The toilet seems to flush normally and the sink drain seems to work properly.

This is a guest bathroom and does not get used very often...I am not sure if that makes a difference or not. I don't see a huge problem here since it's not causing any trouble, but my mother is convinced that one day the house will just float away because I never checked on this. And of course, she is usually right...

I hope I am not wasting anyone's time with a silly question or one that's been covered a milion times here. Feel free to ignore or ridicule this thread as you see fit!

Thanks so much for any help!
-Bess in Ohio

It sounds like the toilet is sucking the water from the p-trap on the lav.
The reason it doesn't always happen, is that the previous flush sucked the trap dry.
If the trap dries out, sewer gas can enter the home.
If the lav is plumbed right, the trap won't siphon dry.
An easy fix is to add a Studor vent like in the picture.

Drain's Clogged

My guess is your drain is starting to clog up.