If the bath drain is gurgling when the sink is emptied..?

[Cooldryplace]

Where is the blockage?

I did have a problem with the bath draining very slowly, but that's cleared now and everything is free flowing.

The bathroom sink also drains freely, in that the water clears quickly, but you can hear it gurgling up through the bath plug hole.

Why and is it a problem?

Following because ours is the same, just the other way around. Empty our bath and it gurgles up the sink. We had all the pipes out and cleared them because it did get blocked at one point, but no issues since, except the noise. Sorry that's not any help!