Plugged AC drain. Eventually you will get water on the floor (yuck!).
The drain is over the transmission, I think. You will need to get under there and gently blow some compressed air up into it, or remove the dirt, etc that is plugging it.
Be carefull, the "hose" is a foam material and you can easily rip it. Replacement may be a PITA, you will have to remove the console on the tranny tunnel to get to it.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!