I too, think it is your sump pump discharge line leaking at a fitting. Pull the insulation, dry framing with hair dryer, add water to run sump, watch and feel for leaks. It could also be an exterior fitting wetting/spraying the siding/sheathing/housewrap under the vinyl siding, running back inside through joint of joist/mudsill (especially is H.W. is not lapped over mudsill). If in a heating climate, you may want to change your fiberglass for rigid foamboard after air-sealing the wood rim joists. http://www.buildingscience.com/docum...l_seal_rev.pdf
On a lesser note; the fiberglass is stopping the uncovered sump well water and condensing there due to the furnace leak at the elbow ducting blowing air/moisture vapor constantly in that bay.