It could very well be humidity. If that's all it is, you are lucky. Allowing the mic to dry out will in all likelihood return it to perfect working order.
If you have some of those little dessicant packages lying around, throw them in the case with the mic. A quicker fix would be to set it up near an air conditioner and direct the air flow so it is GENTLY flowing past the mic capsule. Careful use of a warm air blower (like a hair dryer) may also work - just be careful not to cook the mic (or blast it directly).
Bad/low phantom power is also sometimes a culprit. Switch another mic into the preamp to make sure the phantom is ok. And of course, always check the cable too.
(There's a certain irony that my post #602 is answering a question about a 602!)
