I was not sure which catagory to put this under but here goes...when the central heating is off,my radiators sound as if they are empty but people keep telling me that there is still water in them...so why do they sound hollow when I tap them?
I guess the sound travelling through water isn't so different to that where there is a vacuum inside and the sound is just travelling through the metal of the radiator. (Or were you thinking air was getting in and then escaping again ?)