Funny noise from sink. Help?

[atwing43]

I have one sink in our bathroom that makes this noise when furnace comes on but the other sink doesn't? My furnace is located in attic and someone told me that it's a 3/4 pvc drain from furnace going to the noisy sink. If this is true, any suggestions on how to fix? Thanks!

[majakdragon]

If your furnace drain line ties into the sink drain, it is required to have a trap on it. Otherwise, since it is behind the sink trap, it can allow sewer gas into the attic. Don't really know a reason why it would cause noise though.