Strange dull tapping sound when braking

[audi_do_dat_den]

As the title says, when braking during normal unspirited driving, and when pulling to a halt, I've noticed a tapping sound coming from the offside wheel.... almost like a dull failing cv joint noise.
Only when braking and there's no cv joint noise when on hard lock and no noise when driving in general....
I've had the wheel off and spun the disc, with no wear or cracks.... but the vent holes are stuffed full of brake dust.
Anyone else had anything similar?

[Bogyoch]

Probably a build-up of rust on the inside of the disc.

[audi_do_dat_den]

Checked that and they're perfect on both faces.
They were new last year also.

[iduff]

Maybe some gravel where it doesn't belong? In the tire tread, or stuck in in the brakes?

[johnnydog]

I had a similar issue, although mine had ceramics, so I don't know whether steels have exactly the same set up. I was experiencing a very minor clunk / click (not every trip ) from the front brakes when braking gently at slow speeds (10-15mph). I found that the tension springs from the backing plate of the pads to the caliper had been fitted on the wrong rotational side so that instead of the spring pulling the pad (the movement is minimal) in the rotational direction of the relevant wheel, it was pulling the pad the opposite way, so that under gentle slow speed braking, as the pad grabbed the disc, the pad was moving very slightly in the caliper causing the slight clunk as it contacted the edge of the caliper. I obviously didn't hear it under normal braking, as the speed and road / engine noise drowned out any clunk. After the springs were repositioned, problem solved.
All cars have a system to keep the pad under 'tension' to stop the pad rattling in the caliper when not braking, so I would assume that RS4 steels may have a similar setup to the ceramics.

[Harris.]

I also have a strange noise when breaking , sounds like a squeaky bed spring, its since the rear discs were done . Had the wheel off and made sure the backing plate is all straight etc , very odd

[audi_do_dat_den]

It's actually getting worse now.... sounding like a crunching sound now.
Almost like a mashed up bearing.
However, I've driven today on a new black top tarmac road and coasted for a while with no noise whatsoever from bearings or running gear.....
Annoying to say the least.....