Sometimes a renovation is done upstairs and the workers seal off the vent pipe. When a toilet is flushed air will have to be drawn from somewhere else: from a sink, from a toilet. That's when you hear strange noises.
Other times there is a clog in the line. Someone will flush the toilet and an adjacent toilet will start making sounds. That is the toilets way of telling the homeowner that there is a blockage and a disaster is bound to happen. Don't wait. Call the plumber. Would you rather the waste back up first onto your house floor before you decide to call?