Normally when you flush your toilet, the other drains in the bathroom do not make noises. If your bathtub's drain makes noises when you flush the toilet, that indicates the bathroom's drain system is not working normally. Ignoring this issue can lead to larger problems in the near future.
When you flush your toilet, the water flows down the drain pipe in the floor under the toilet, which eventually meets up with a larger drain pipe shared by other plumbing fixtures such as the bathtub. When flushing one drain causes noise in another, air in the drain pipes is to blame. You must then find the source of the air and take corrective action.
While not highly likely, a clog in either drain can lead to trapped air and noises in the drains. Normally the clog will cause the sound in the clogged drain when you flush water down it. Still, plunge the toilet and even feed an auger down the drain to eliminate a clog as a possible source. Plunge the bathtub's drain and feed an auger down its overflow drain as well. Flush the toilet again to see if the same noise continues.
All of the drain pipes in your house connect to vent pipes, which serve dual purposes. The vent pipes give a safe passageway to expel the sewer gasses trapped in the drain pipes, keeping the gasses out of your home. The vent pipes also bring fresh air into the plumbing system, replacing the air that is sucked out of the drain pipes along with the waste water. A blocked vent pipe will cause problems with the drains in your bathroom or other parts of your house.
Clearing Vent Pipes
Clearing the vent pipes yourself involves climbing onto the roof. If you have a fear of heights, consider paying a plumber to clear the vent pipes. Once on the roof, look down the vent pipe over your bathroom using a flashlight. Use a drain auger to dislodge clogged materials like leaves or a retrieval tool to extract large solid objects like tennis balls. A hose with a high pressure attachment will dislodge clogs and restore the vent pipe to its normal functioning order.