Try some copper anti-seize grease on the back/edges of the brake pads and the retaining pins where they touch the caliper and piston.

Dixcel do this down your way.

Do not get any on the friction surface.

The idea is to isolate parts as the whole brake assembly is resonating as you brake.

If this fails try another make/compound of brake disc or you amay be able to get ones with anti-squeal shims that sit between the back of the pad and piston to prevent this problem.

Hope this helps.

Sandman.