My 2001 had the same thing, it was a bad heater core that leaked into the HVAC system and sloshed around. You only have to take the entire dash out to replace the heater core, DAMHIK!
The funny thing about science is that if you change one miniscule parameter you change the entire outcome to the way you want it.
You only have to take the entire dash out to replace the heater core, DAMHIK! 
