when you say resealed the plenum are you talking about around the fresh air ducts? the round hole in the plenum that those ducts screw into has a lipped edge and the duct goes through and sticks into the plenum it has a foam gasket that seals it all up, if theres no rust there it shouldn't leak it is common for rust to form around it especially around the screws that hold the duct up, if you use silicone to seal the duct it makes the rust worse and makes the leak worse (it is a short term fix)