Leaking under neath heating unit but it's not the hose but a little white knob underneath unit

Ac drain . There could be a blockage in the AC drain pipe that runs under the car. This can usually be unblocked with compressed air they jack up vehicle takes 30 seconds to fix with vehicle hoist.
When it blocks up the AC unit builds up with water and leaks clear water inside the car. if it was green water it would be the heater core leaking coolant.