toilet is burping – HELP

Hello there everyone…I am hoping someone out there can help me. I have only one bathroom in my house. When I turn on the bathroom sink, after about 30 seconds, my toilet starts to burp, but only for about5 seconds and then it stops. If I turn on the shower, the same thing, but for a few seconds longer. If i try and flush the toilet when either the tub or the sink water is running, water fills in the toilet to the brim, NEVER overflows and will then go down slowly once the water from either place is turned off. Septic tank has been pumped, there appears to be no blockage in the vent thru the roof. We had a plumber come in and he himself is stumped because all 3 separatley drain and flush fine…everything goes out the way it shoud and nothing comes back in and never has. The plumber said the only thing he could come up with is that there is a galvanized pipe that goes from the shower, connects to the sink and vent pipe and connects to the 4″ pipe for the toilet (which he said isn’t the way it should be to begin with) but that the pipe is probably rusted form the inside and therefore restricting the flow of water and air going thru it. Air must escape from somewhere so as the water comes down this pipe and goes into the 4″ toilet pipe, the air is coming up thru there because there is no room for the air in the galvanized pipe above the waterflow. Does this sound possible and make sence? He is suggesting that we remove the galvanzied piping and replace with PVC and this should hopefully fix the problem. I would appreciate any feedback possible. Thanks for reading.