Hello, I really need some help on this one folks as I'm rapidly going bald tearing my hair out!!

I've got a 2002 ST170 on 42,000miles, with a seriously annoying knock. It sounds as though it should be a knackered bush, like a bottom arm, but I've had a good look at the front suspension and nothing's split, broken, worn or loose. It doesn't knock if you rock the car on the handbrake, or if the you floor it and let off, there's no play or slap in the steering either. If I drive over a cobbled street it sounds like chattering teeth!! However, if I ride the brake pedal the noise stops. I've checked the brakes and the pads are a little down, but they're not on the metal, and ideally the discs need swopping when the pads are changed but they're not exactly razor blades. The pads seem a really good fit in the caliper (had to tap them out with a small persuader lol), because i thought it might be them, rattling.

Anyone come across this before?