Our toilet "groans" when flushed- it makes a loud noise that does not appear to be correlated with anything I can see. Also, it randomly bubbles and leaves black pulpy residue inside the bowl. Is there an easy fix to this that I am missing?
Check your upstream vents. Drain cleaner or a drain snake usually does the trick! If the vents are okay, you may need to call a plumber.
If you're on a septic system it might need to be cleaned.