I know there's a few other thread on here that are similar, but none were quite the same. We've been renting a house from friends since May, and within the past couple months we've had the toilet overflow 3 times, we've found the tub with black gunk and water backed up into it (not sewage), and several times have heard the toilet gurgle while showering. The first time we noticed the tub backed up with stuff, we were getting ready to leave and I think we both had just showered, the second time we came home to it. Before we moved in my friends had mentioned they had problems like that, but it would happen in spurts... everything would be fine and suddenly it would happen a few times and then it was OK again. Shortly before we moved in, they had RotoRooter out here, and the workers weren't able to find anything wrong. Today we had the toilet gurgle again, and my hubby told me to flush it and see if it stopped (we don't know anything about plumbing obviously) and the toilet overflowed. He plunged it, and the toilet eventually drained, but there was grayish water backing into the tub. He flushed the toilet again to see if it was cleared, and the same thing happened, except water and black gunk backed up into the tub again. There's never been any kind of odor to the backup though. We've also had a few issues with the tub not draining while we're showering. We live in the city, so we're on their water, not well (don't know if that matters).

Sorry for such a long post, I just want to make sure I explained everything right. Also wondering if this is going to be a big issue, as we are considering purchasing this house but don't want to buy a huge problem either. Thanks.

City sewer or septic tank? You should know that this problem should be fixed by the owner even though they are friends of yours. Friends maybe but with rent paid they are the landlord and responsible to provide habitable housing, by law.