CRT monitors 'pinging' (make a noise and flash off)

Both my flatmate and my girlfriend's monitors 'ping'.

What I mean by this is that when they are on they will make a pinging noise, the screen will go black for a second and then come on again - as though it had been turned off and on. I thought it was only to be expected with my girlfriend's monitor - a very old IBM one (6546 if I remember correctly), 15". However my flatmate's monitor isn't even a month old and it's doing it just the same - his is a Hyundai Imagequest 17" flatscreen CRT. He claims right now that it does it every time he deletes something (?).

Does anyone know what this is? Or if there's a way to fix it?

Thanks.

Some models have an auto degauss on them, could be what is going on.