water leak through heater??

I am wondering if water will enter the passenger compartment through the heater air intake if it is raining real hard?I do have some water entering on the passenger side of my 99 suburban.It has happened before,but it has just happened again.Windshield has been in the vehicle for a few years,so I am discounting the windshield.This is why I am asking about heavy rain fall entering through the heater air intake.Is there a drain hole some where that may be plugged from leaf debris or such that is allowing the rain water to accumulate and then overflow into the heating system??