Tom is 100% correct it is vent problem; check the main vent pipe. You quite probably have an obstruction which is giving you the soda straw effect. Put a straw in a coke, put your thumb on the top and remove it. The water will remain in it until you release your thumb (obstruction) and equalize the air pressure. IF you feel comfortable doing it, take a garden hose on the roof and chug the vent pipe until the obstruction is eliminated. You would be surprised what squirrels put in pipes, If not follow Tom's instruction, Good luck.
John
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