knock noise from n/s front. what is it?

[20VT-Neil]

i'm getting a very noticable knock noise from the n/s front, this is when i apply the brake, not when I'm going over bumps, as I know if it was doing it on bumps i'd think it was one of the arms,

This is something that needs looking at, but need some help on this.

I've allready had the wheel off and cheeked the caliper bolts and they are tight so can't be that, is there a common problem that i don't know about?
Let me know what you think, any info would be great.