The answer to 'why is my bathtub making gurgling noise' is that it's likely caused by a blocked or clogged drain. Gurgling sounds are a common sign of a drainage issue, where air is being forced back up through the drain as water tries to flow down.
Some common causes include hair, soap scum, or other debris buildup in the drain, a problem with the plumbing vent, or a blockage further down the drain line. Gurgling can also indicate a problem with the bathtub's P-trap, which is designed to prevent sewer gases from entering the home.