I have what I think is a weird problem. When I flush my toilet, there is a gurgling sound in my bath tub. When I run some water in the tub,and then flush, the noise stops, for a bit, (3/4 flushes) and starts again. Even more weird. this only happens on an intermittent basis. Sometimes it will go weeks without happening, then do it for a week. It seems that for some reason the toilet is sucking the water from the trap for the bath. I thought it might have something to do with the snow on the roof covering the vent stack but I just checked it from the ground) and it is open. Could there be a plug of snow inside the stack pipe that I can't see from the ground? (stack should be warm enough to prevent that?)

Any thoughts/ideas would be most welcome. This is very annoying!