Generally speaking, a leak big enough to hear will usually fubar the idle.
However, maybe in this case it don't for some strange reason.
A really nifty "Grease Monkey - Old Timer Trick Quick and Dirty Vacuum Leak Test" is to use a portable propane bottle and spray UNLIT propane in the general area where the noise comes from while the car is idling. When the idle goes up, you've pinpointed your leak.
Good luck!!!
TomC
"Crabs"