About 4AM this morning my wife awoke hearing a bubbling/gurgling sound coming from our master bathroom toilet. (no one had been in there.) I went to the other toilet (between it and the street) and found it bubbling also. In a little while, the water level went down, but each flush OK, though the master bath toilet seemed to refill slower (hopefully not related.)

Last year we had someone rotor out the pipes due to a problem, but I am thinking this might be from the street side. Is this possible? How do I start without calling a plumber (first?)