Do, or rather did, you have a chicken wire mesh cover over an outside
vent pipe?

If so they tend to fall in eventually and cause a restriction.
Toilet paper breaks down readily, except for Andrex & some others,
poking a hose will easily degrade its integrity and it will go (indeed
with a whoosh). The blockage can remain above the elbow in the vent
pipe (vacuum).

Bang on the soil pipe and you can often hear the old mesh chink as it
crabs downwards.


Might be worth sticking a sacrificial camera down if it re-occurs,
plugged in of course :-)