i have a downstairs WC with a short length soil stack, built into a boxing cavity in the wall. When the WC is flushed and the system has emptied the soil stack shakes quickly side to side for a few seconds.
If i take the air vent grill off the boxing cavity you can see the top of the soil pipe and its own air vent lid. its the very top of the soil pipe that shakes side to side.
The WC doesn't show any signed of blockage and other WC in the house are fine, although they run on their own soil stack.
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