Check the axle vent hose, normally on the drivers side of the axle tube with a hose that goes up to the frame and makes an inverted trap. If that hose is missing/disconnected, you may be getting rain in it. I would pull the cover off, spray it with a full can of brake cleaner, it will evaporate fast enough where you could put the cover back on the same day.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy