Fiesta Zetec whistling noise when decelerating.

My Fiesta has developed an erratic (i.e. hunting, almost stalling) idle.  There is also a whistling noise which has developed recently when decelerating.  At the moment, I'm going to assume this indicates an air leak.  Since it happens when decelerating (i.e. when the throttle plate is closed), I'm going to assume it's not something nice and simple, like the air filter box.  I've tried a smoke test, and haven't seen any coming out (or loss of pressure), but I suppose it's possible that the leak (if that's what it is) only appears under vacuum, and doesn't appear under the positive pressure of the smoke tester.  I suppose it could even be whistling past the throttle plate itself, but if so, why haven't I noticed it before?  Before I start taking things apart (e.g. brake servo, induction manifold), I thought I'd ask if anyone here has experienced something similar?