Ken: Sounds as though your basement air, is especially warm/humid and probably fairly static above your basement ceiling. Do you have any 'slight rust' problems in basement are. For example do metal tools show very slight rust after few months or any similar indications? I know that we humans breathe out quite an amount of moisture and if people sleep down there it will be a for a significant percentage of the 24 hours. Can't remember the amount we breathe out but is it something like about a quart per day? Quite apart from any sweating we do? To have condensation like that on the cool air return pipe from upstairs or wherever the return comes from, means that the returning air is cooler than the warmer and apparently much more humid air surrounding it above your basement ceiling. Maybe the upstairs is air conditioned so that its air is cooler than the air in the basement? Is the basement sufficiently ventilated or do you need to decrease humidity down there by use of a dehumidifier? We use a dehumidifier in part of our basement. Got one of those instruments you hang in the hall to tell the humidity and barometric pressure? If so or can borrow one you could measure the relative humidity down there in comparison to upstairs. Probably won't be very accurate but will give you some idea of the relative humidity. That's the problem; whenever warm humid air cools down the moisture in it condenses and turns into water! That's how a dehumidifier works, why fridges have to defrost (or be defrosted) and that's how rain is formed. Air conditioned air will also have lost some of its moisture just by virtue of being cooled! Few ideas anyway.