01 Sienna - Smelly AC Problem

[jman_va]

Starting last year somepoint, my Sienna developed a rather odd A/C problem.

Whenever I turn on the A/C for the first time after the car has set for a few hours or overnight, the A/C for about 5 min smells awful. The best I can describe it as is a musty gym-sock. It does not have any sulfouros / rotten-egg smell, but more of just a sweaty, musty old gym-sock.

The smell goes away completely after opening the windows and letting the A/C run for about 5 min, but it is an eye watering / almost gaging 5 min if you leave the windows up. It is also embarrising to say the least when you have guests in your car.

It does not matter wheather the A/C is on recirculate or fresh air, the first few min are always funky, but it does always clear up. After the smell goes away and I turn off the car, if I return within an hour or so, it does not smell when turning it back on, only when left off for a few hours or more.

Ideas??? Thanks!