Lately I have been hearing a gurgling noise coming from the toilet bowl and can also see bubbles coming from the bowl. This happens mainly when either the kitchen sink is draining or the washing machine is working and draining. The toilet water level in the bowl will start to go down and if it is flushed the water will rise and almost overflow. After I use the plunger the water level goes down very slowly. After a period of time the water returns to its proper level and the toilet will flush with no problems time after time if the kitchen sink or washer are not going. Is there anything that can be done before a plumber has to be called out? I'm a DIY kind of guy but this has me baffled.