Thanks for your advice. I checked the roof out and sure enough, there are two vents that emerge from the roof.
It sounds fairly easy to unclog it so I'm going to buy a snake this weekend and see what happens. I'm assuming I can use a flashlight to try to figure out which vent is clogged...
Any other advice would be welcome. Otherwise, I'll give it a go and let you know what I find. Thanks again!
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