The toilet vent pipe in my mother's 60 year old home howls whenever we flush. We can hear it outside, also as the pipe goes up the outside of the house. The toilet also bubbles whenever flushed. A plumber says the vent pipe could be partially plugged. It's not affecting the process, but is annoying the neighbours at night. It moans.
Your plumber is right on target. It is very possible for something to be down in the vent stack i.e.: a bird or squirrel carcass or possible a build up of leaves etc. By using a garden hose and a drain snake you or the plumber should be able to release this obstruction from the top flushing the toilet at the same time.