Plumbing Question
 

We have just noticed this in the last few days. When either toilet is flushed, there is a gurgling, or low rumbling, sound coming from the drain in the guest bathtub. I was wondering if anyone has experienced this or has any idea of the source.

Check your vent stack.

I don't know what the problem is, but the first thing I would do is to make sure the tub drain (and all drains) have water in the trap. Traps can dry out. You could also try to plunge out the toilets to make sure they are not clogged. If these are not the problem, then it sounds like a venting problem, and you probably need a plumber to fix it. All fixtures vent through a pipe on the roof, and they can get clogged up by birds, debris, or other animals.

When that has happened in my house, the main drain has had a partial clog and needs to be cleaned out. If I don’t get it done, we get water backing up into the shower and tub.

Thanks for all the suggestions. The tub had not been used in quite a while so I ran some water, and used the plunger on the toilets, for good measure, and the noise is gone.

Check YouTube if you have not. I do not know the answer but I would fill up all the sinks and tubs with water and open the drains at the same time. Also turn on the faucets. Trying to flush out the lines. If you have a plunger use it on the tub giving you the problem.
My guess is you have a vent problem but the above is easy to do so I would try it.