Mr Magoo, I assume the gurgling sound that your parents hear is when they drain the sink and the bath gurgles, they drain the bath and the sink gurgles or there is water being drained from somewhere else in the house and the sink and bath gurgle. If so the problem is that any of the above situations causes a syphoning affect by the moving water. If they can live with the gurgling there should be no problems but there is remedies to the problem - have a look at the site below...
[broken link removed]
The one problem that I can think of is that the syphoning action would drain the water from the S-trap under the bath/ sink which would allow smells into the house.