When the bathtub is full of water and water is overflowing into the overflow valve, there is a knocking or dripping noise. The plumbing under the tub is visible from the basement and there is no sign of leaks. What could this knocking sound be?

Hi Snbuck...

That sound is the sound heard when the water drains out the tub vent, if you will, instead of the tub drain... ;)

Normally, water that drains down the tub drain mixes with air from the tub waste overflow pipe and flows into the pipes where it mixes with air through the tub vent and produces a turbulent flow to the water and actually self-scours and keeps the tub drain cleared. The volume of water is such that you can't really hear the water draining, or at the least you don't question the noise, right?

When you have water that flows down the overflow exclusively you end up with less air mixing with the waste water and the drain kind of glugs/chugs and makes the noises you are hearing because you can hear them and notice it... that's all!

It could also be indicating a small vent issue, but I don't think that is the case here... most likely as I said above if the tub drains normally, OK?

Questions? Let me know...

Mark