I have a 5-yr-old house with an odd plumbing problem. What happens is that if I or my
wife runs the water in the sink in the upstairs bathroom, air gurgles up through the toilet.
And it seems to be just air, there's no odor to it. Nothing happens when we run the
shower.

At the same time, we sometimes have weird problems with the upstairs toilets (there's
one other bathroom upstairs). For no apparent reason they'll both get plugged up at
once.

I tried using Drano on both upstairs bathrooms but it didn't seem to help. Anything else I
should be doing?