Very strange PS1/PS2 problem

[Martin]

I notice that when playing some games on my Pal Hitachi TV I get a flickering at the top of the screen, it happens on all my PS1 games, and a handful of my PS2 games.
At the same time it also produces a high frequency whistling sound when the flickering occurs, like a "hiss <break> hiss <break> hiss".
any ideas what's causing this strange problem?
I notice I don't get it when running in NTSC 60Hz mode.