my guess is the fall of the flow out is greater than 1 in 40 and if other wastes are connected (sink, bath, shower.) Check if any signs of loss of water in these traps usually by the small.
What happens is the water moves so fast it creates a vaccum behind causing the gurgling and some times sucking out all of the water from the other units waste fitting. Fitting air inlet valves can eliminate this a good plumbing show will tell you how and where in the pipe line to fit them. If not I'm sure doing a Google search would bring up 'how to fit air control valves in waste pipe.
bob UK, June 2014
