If your 95 is stock, it has multi port fuel injection, so no carb. The vacuum for accessories is drawn from the intake manifold.
The position of the fan speed switch should have no bearing on a vacuum leak as the fan is electric. If it was vacuum/heater related, it would start acting up as soon as you moved the lever from off to defrost, heat, etc. regardless of the fan speed.
With a leak that big, it should be easy to find. Do whatever you have to do to duplicate the sound & try to track it down by ear.