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I'm thinking the same thing as Wrench. The vent is probably cracked below the roof sheathing in the attic, so you should climb in there and take a look, or research doing the peppermint test like Wrench said to pinpoint the leak. A professional journeyman plumber in your area will have scoping equipment to check for a break in the stack, so that would be your best bet. Usually when the right person looks at it you can clear the problem up quick. Good luck!