Ford Mondeo noise from brakes when braking

I get a whuppp whuuuppp whuuuppp noise form my n/s front brakes when braking. Noise is not the same as worn pads but is just as loud or even louder. It is only audible for about the last 50 yards or so of braking to a halt and as soon as you take your foot of the brake it stops. Ford garage are baffled (whats new) and have changed discs pads and caliper. The noise is only there in forward motion. It doesnt happen when braking in reverse. The noise goes off when the brakes have been taken apart for new pads etc or goes off even if you take them apart and dont replace anything but comes back on again within a day or so. The longest it stayed off was when the caliper was changed and lasted about a week without making the noise. Any suggestions please. Thanks.