Hi - I live in New England, and my A/C system is in my attic. I replaced the A/H and Coil a couple of years ago with a Bryant Evolution system, but we kept most of the original (1987) duct work (insulated 8" flex ducts off a common insulated main rectangular duct). The static at high-speed is about 0.6 if you're interested.
This weekend, I decided to remove an in-duct fan that I had put in years ago for a particularly long run but haven't needed since I put in the Bryant system. No problem, except I looked in the flex duct for this run and, as far as i could see, there seemed to be an intermittent coating of a damp, greenish mud going off into the duct. I don't think it was mildew because mildew usually adheres and this came right off onto my glove, but it wasn't pleasant to see.
Now, admittedly, it appeared that I hadn't done a great job insulating the duct fan, as after I pulled off the insulation i wrapped it in, the fan itself had condensation on its outside, but the "mud" trail went way into the flex duct, as far as I could see.
It's a 20+ year old system, but still, is that normal? dangerous? don't care? and if it's bad, what should I do?
/j (who needs to stop looking for trouble)