Whilst braking moderately from high speed, after a second or so, I get this rapidish oscillating hum from the front brakes. It's not noticeable through the pedal or as vibration, only sound. Braking efficiency doesn't seen effected and if pedal pressure is eased a little then it goes away.

05 WRX with braided hoses, padgid 421's and motul fluid. NOTE however I also noticed this before these brake upgrades were done and it's something that has been happening for some time (over 12 months)..

IMO it is either a warped disc or possibly a trait of the ABS..? The thing that's puzzling me is why it only happens under prolonged moderate braking and not light or even very heavy application..???

Checking the run out of the disc's would be a start, but I don't have a suitable dial micrometer..

Any advice/comments appreciated as usual..

 

Just some thoughts here .... A warped disc normally causes the brake pedal to pulse and if the disc is warped it should do it at all times.

I've known of strange vibrations when braking caused by a poorly secured solid brake pipe under the car/in the engine bay vibrating against the car body.

Wheel bearings with play in them can cause random brake related noises/vibrations which can be there one moment and not the next. Thisis due to the tolerences changing as the bearing heats up and the clamping action of the pads gradually restricting the bearing play as brake pressure is increased.

Could be pad chatter at the specific brake pressure you mention?

And other potentially less likely areas could be worn suspension/wishbone bushes/track rod end.

Probably other things but the above will keep you busy for now

 

With regards the warped disc, I have the same feelings, in that surly I'd feel it via pedal..?

Suspension ok, I'm running AST coilovers and the noise occurred before and after having them fitted.

So of the things you mention two stand out.

Pad chatter is a possibility as the Pagids I'm running do clunk/move when I brake after reversing, but then again this vibration noise occurred with the OEM pads which didn't move in the caliper..!?

The other thing, and this is very possible, is a brake pipe vibrating against the bodywork somewhere. This I will have to investigate..

Cheers,
Jason