It might be a fault in the fill valves, it may be a clog creating negative air pressure, instead of flowing through the pipe freely, the air pushes it back causing a bubbling/gurgling noise. You may see the the toilet water bubble which means the toilet or drain line has a blockage? It could be a build up of calcium on the internal bore of the pipe or something similar?

Contact serviceteam to get your toilet unblocking done for an affordable price.