Thanks for the replies. Appreciate you trying to help me out.

Humidity has ranged from 38% (very cold and dry outside) to just past 50% today (warm and humid outside), normally (over the last four weeks) 44 to 45 percent. I read where humidity past 70% can be a problem. I have a dehumidifier waiting in the wings, but thus far haven't needed to hook it up yet. Temps range from 68 degrees (lights out overnight) to a high of 77 degrees (nearing end of light cycle).

The setup is three adjacent sealed rooms, each 8 by 8 by 7 ft high. Cool air is drawn from one end through a 12 by 12 register, then through a series of registers passes through the middle and then to the end room. I have doors between rooms, but haven't had to isolate anything yet, so the doors are open and the airflow passes freely from room to room. The end room has the filter/fan setup. I added a pair of home-made mufflers to the intake and exhaust to deal with the noise of the airflow. I am pulling through the filter. The rig is mounted up near the ceiling to exhaust the warm air out. All metal ducting seams in the elbow and my mufflers are sealed with metal foil duct tape. All connections are properly secured with hose clamps. For what it's worth, I worked with flex ducting a fair bit during my days as an aircraft mechanic, so I was taught how to do it right.

The arrangement is - carbon filter, six inches of six inch insulated duct, muffler, 45 degree elbow, 18 inches of insulated duct, inline fan, four feet of insulated duct passing through the wall, exhaust muffler. The fan is about six inches from the wall; where the duct passes through the wall the hole is sealed around the perimeter of the duct.

I'm somewhat reluctant to name names, as I don't want to slur the supplier should the problem be my own, but I purchased the fan/filter from High Tech Garden Supply. This is their GrowBright 6 inch fan and filter combo. While they sell Can Fan filters, I don't think the one in the combo is one of them. If I remember correctly, there was no mfr name on the box the filter came in, and there are no identifying marks on the filter itself. The filter is supposed to be a "packed bed" design. It does come with a prefilter, which was installed when the filter was hung up. Link --> High Tech Garden Supply

I've wondered if the mufflers I've added are a problem, but cannot fathom why this would be. I detailed the muffler construction in another thread a few posts down --> http://boards.cannabis.com/growroom-...-proofing.html

Been thinking this up and down and sideways, and am drawing a blank. I could see an unsealed room emitting odors, but that's not the case here. The smell is definitely coming from the fan exhaust.

Going to call them tomorrow and see what they can tell me. Have to find a fix very, very soon.

Edit - the ozone filter is something to think about. Thanks for that.