My guess is that you have a drain line that is partly clogged. You can try to auger it out yourself but I think you will be wasting time and money. I would call a drain auger plumber and have him fix it, before it gets worse. If there is also a problem with the vent, he could fix that at the same time.

I do a lot of small plumbing repair, but I don’t do drains. It takes a powered auger and you have to know how to use it. Also messy work!