I've seen that happen before and with the cars I've seen, it was a clogged PCV system, excessively high engine vacuum and cured by removing whatever was clogging the PCV system.
Try this, when you hear the noise at idle (engine running), slightly crack open the oil fill cap. If it sounds like a soda can opening, pop type noise but sucking air IN (vs under pressure pushing air out, yet another problem), you then still have an issue where your PCV system is not flowing correctly and they didn't fix whatever's causing the restriction.