Toilet glug and vibration due to new loft conversion?

[moveonup]

We have just had a loft conversion completed with an en suite bathroom in the loft.
Since we moved up into our new bedroom we have noticed that when we empty the original bath in the bathroom below on the first floor it seems to make the toilet in the new en suite in the attic vibrate and glug.

I have discovered I can stop it doing this if I leave a toe or the plug half in, over the plug hole while I empty the bath. This seems to slow the flow and stop the glug vibration.

Any ideas what is causing this or any way of solving this. Is this something the plumber from the loft conversion company has done wrong??

Thanks

 

[wiss]

Is there a vent on the new soil pipe up in the attic?? sound like when you are draining your bath is creating a vacume in the soil pipe pulling out the water in the toilet u bend,

 

[moveonup]

It originally it went out the roof, but it was been cut off and I guess, hope some kind of durgo valve has been fitted. But I can not be sure.

 

[ChrisR]

A durgo only lets air IN, you had a vent which let it in AND out before.
I'd check with the Building Control people, who presumably inspected?