Wayne from Prince Albert, Saskatchewan writes: "I hear a constant dripping in my 4 inch sewer stack that connects my upstairs toilet drain to the floor sewer. It is not coming from the toilet but since the venting goes to the roof it is possible that during the winter, warm air is rising, condensing and dripping back down the stack?"
Wayne, I question your conclusion that it is not coming from the toilet. Yes you can get condensation and frost at the top of the stack, especially in Saskatchewan. But when it melts, it melts around the edge of the stack and generally flows down the walls of the stack, making no noise. To really see if the toilet is responsible, turn the water off to the toilet, flush it and check that it does not fill back up. Leave it like that overnight and listen for your problem noise.
The reason I suspect the toilet, is because either a very slightly warn flapper or a slightly poorly adjusted shut-off valve can feed water into the system very slowly, providing the drips you need for that noise. In addition, if you look at the second graphic, it is easy for some kind of partial clog to have extended the lip of the horizontal drain line into the centre of the plumbing stack. This would allow that slow water flow to literally drip into the centre and drop a great distance before striking the wall or even the bottom of the stack, making that annoying sound that you don't want to live with.
Stopping the leak in the toilet and snaking out the line from the toilet to the stack should eliminate the sound.