Recently, I noticed that every time I flushed a toilet or ran water at a bathroom sink, I'd hear a loud gurgling noise in the bathtub. Of course I checked the net for what it could be. The most common answer was that our roof sewer vents were clogged up somehow. Roof sewer vents are open pipes through which air enters the system. They allow dangerous gases to escape the system so the house doesn't blow up.
The next most common answer was even more dire: a secret clog in our pipes, and the net warned that using a drain cleaner would only disguise the problem. However, the problem being on the roof was the most cited answer, so I attempted to find a solution.
I called a handyman company, but it declined to go up on the roof and look at the vent. I called my usual plumber, but they said hey don't do that sort of thing. The only company anyone recommended was a big plumbing company that does everything--a company I know charges a fortune and which from experience I believe does not offer intelligent solutions, only expensive ones.
Changing out the roof sewer vents would compromise the roof itself and the risk would be a leaky roof. That could result in catastrophic damages during heavy rain or after snow or ice.
So...I waited for better weather to call them. Also for more money to magically appear in my bank account via a tax refund, because I couldn't see this company putting someone on the roof and replacing a sewer vent for less than many hundreds of dollars. Call a plumber and two show up, so whatever hourly fee is quoted is automatically doubled. A roof repair could be complex. This is how plumbers get rich.
A couple of days ago, I ran some water in a bathtub that is only used when guests come to stay. Now the sounds are gone. Surprise!
Conclusion: The internet was completely wrong about the origin and meaning of the sounds. Every site I visited said the same thing, a clog of some sort. Turns out the origin of the sounds was a lack of water in the p-trap under the seldom-used bathtub. Now that there is water in the trap, the sounds are gone. I dodged an expensive bullet.
I hope this information helps someone else.