My blower runs 24/7 in low speed, and goes to high speed when the compressor kicks on. When it does, we smell a icky smell for a few minutes. It happened last year also, so I cleaned the Acoil. But after thinking about it, I figured that since the fan is running full time, there wouldn't be any residual dampness or mold.

What I think may be happening if it is possible is that when the fan goes to high speed, it somehow pulls air up through the condensate drain and we smell the foul air. The drain is in a "Y" with the humidifier outlet (aprilaire 760). It could pull air down through the humidifier part of the condensate drain and up into the Acoil. There is no S or P trap inline. I will try to post a photo to show the anatomy of the plumbing.

 

and pull the blower, have seen many a dead critter lurking there

 

Put the P traps in for both the ac coil and the humidifier. You do have the water to this humidifier turned off I hope. Like hvac4u said--- "them little critters do get in some times" ED