I have Level II NDI cert and yes, the tap test is valid. We use it on aircraft, a simple quarter and the human ear is decidedly one of the best ways to find delaminated areas or voids in composities. The difference in sound is easy to pick up. Of course we use other methods to delineate the damage for repair.
With a coin, tap the tube starting above the damaged area and continuie across the damage, if there is a change in tone that area is higly suspect.