Wierd smell from vents

[randrews@post.cis.smu.edu (Robert Paul Andrews)]

My 93 90 just starting making a wierd smell from the vents with the 
blower on.  Sorta like the smell of sweat or something old.  Not electical
or fluid.  It only happens when the blower is on and AC off.  When the
AC is on, it goes away.  maybe mildew from the water evaporation?  
Anyone else experience this?
Later!
Rob