Virtual Cow

Hi
I've got a strange noise coming from the front brakes sort of a rattle or
rumble like noise and fairly loud.
This only occurs when applying the brakes hard at higher speeds 100kms to
120kms.
The dealer told me it was warped disks so I had pads and disks replaced also
wheel bearings , the brakes where checked and the problem still occurs.
Note the car has no ABS so its not the anti skid.
There is no other noise or vibration when this happens.
The dealer does not want to test the car over the speed limit and has not
heard the noise only my limited discription.
Any ideas ? before I go back to the dealer and get "taken for another ride"
Thanks