Ultra-sonic; I've got one and it does work, in some cases. It is astonishingly effective for steam or compressed air leaks in plant-rooms, where the sound produced by the leak can be detected despite the background noise.
You'd need to blow all the water out of the low-lying sections of pipe, to allow the air to escape from the hole. You shouldn't pressurize it above the current working pressure; more pressure doesn't make the ultra-sonic sound louder. 5 or 7 psi/ 0.5 bar is adequate.
The problem is that the ultra-sonic or high-frequency sound is most easily absorbed and attenuated and is made inaudible by any covering or obstruction. It needs to be on a direct line of sight. Conversely, when someone has a party at 3 a.m Sunday morning, the low-frequency bass goes for miles and is often the only sound you can hear.
Ultra-sonic is not much use under floors.
