Drip Sound in Bathtub Plumbing

After running hot water into bathtub and turning water off (with bathtub drain closed), we hear a drip.. drip... drip in the wall. It actually sounds more like 'thump'... 'thumb' ... 'thump'. This can go on for several minutes, faster at first (a drop every 10 seconds), slowing to a drop a minute, and then finally stopping after a few minutes. But there is no evidence of leaks, and we are certain there is no water visibly dripping from faucet or shower head. We surmise that if the water was leaking from a pressured line, that the drip sound should be present all the time, i.e. rather than just for a few minutes after running the water. We surmise that it is not the bathtub drain because it remains closed with stopper installed.

What could be the cause of the dripping sound?

Could presence of air in the water lines, coupled with bends in the lines, somehow cause a drip to occur 'inside of the line'? Or is there another way a drip could be occuring?

Douger

Turn on the hot water faucet and let it run. You will hear the "drip". IF you listen closely, you will also hear a quieter version of the sound as the pipes cool down. Turn on the cold and let it run, you will NOT hear the drip. IT is the pipes expanding as the hot water flows through them. There is seldom a "cheap and easy cure" for the problem.

This is more common in the pvc drain lines, but out of curiosity, do you have cpvc supply lines? Both the plastic and metal lines expand and contract with temperature, but the plastic ones change length greater than the typical copper ones. If the holes the pipes go through or the clamps holding them are not installed well, they can rub and make that sound.

IT is more common with copper, because copper cannot flex to absorb the expansion like plastic lines can. Since this person hears it when the water is running, rather than while draining, it appears to be a hot water pipe causing the sound.