An educated guess would be a vent problem. Plumbing drains need air to work properly. This air is supplied by the vent (the pipe you see sticking out of the roof). If it is clogged, when another fixture is used, it tries to find air, and if it can not get it from the vent, it sucks the water out of another fixtures trap. This is where the sound originates. If it is safe to do so, go onto the roof with a flashlight and look down the vent for obstructions. remove any you can by hand and then flush the pipe with a garden hose.
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