[pumpo64]

My i30's suffering from an occasional knocking noise from (I think) one of the front wheels or driveshafts.  It's a repeated knock once per wheel revolution, only noticeable at low speeds and regardless of what gear the car's in.

The mileage is about 38,000.

It's as if some debris, like a twig for instance, has got into the vent gap in the front disc and is hitting the caliper with each revolution.  It lasts less than a minute and then goes away completely.  We called the AA once, but their mechanic could find nothing wrong as the noise had disappeared.

I've taken the wheels off and spun the discs.  There's plenty of material on the pads and the front discs measure 22.3mm (right) and 22.5mm (left).

Has anyone else experienced this?  Any ideas?  Can pressure sensors come loose inside the tyres?  Is it possible one of these is rattling around intermittently?

[nzenigma]

occasional
 from (I think)
 one  ??
 front wheel
or driveshaft.
 knock once per wheel revolution
 only low speed
 lasts less than a minute

quite good explanation thanks but at this stage it tells me nothing.
sensor in tyre would remain.
possibly a CV is a bit dry.???
Does it occur when turning?

Wouldnt worry, wait for it to develop further. When wheel falls off , at least you will know which one it is.

[pumpo64]

Thank you nzenigma.  No, it doesn't happen when turning.  I experienced it last night, after braking in a straight line, which makes me think it could maybe be a sticking brake caliper, plus a very slightly warped disc.  Anyway, I'm going to book it in to a Hyundai dealer for assessment.

[tw2005]

Thank you nzenigma.  No, it doesn't happen when turning.  I experienced it last night, after braking in a straight line, which makes me think it could maybe be a sticking brake caliper, plus a very slightly warped disc.  Anyway, I'm going to book it in to a Hyundai dealer for assessment.

If you have warped discs you can generally tell. If it's the front often you'll feel the steering wheel shimmy on application. It can be more noticeable with hot brakes and vary depending on speed and brake pressure if it's only slight. If it's bad you'll have no doubt.

Rears you may get a pulse in the pedal or just as you slow with even pressure you'll notice the vehicle unevenly slowing down or pulsing just as it comes to a stop. Braking is generally less efficient with warped rotors.

Can't hurt to check for runout and if the guide pins are seized.

[nzenigma]

Thank you nzenigma.  No, it doesn't happen when turning.  I experienced it last night, after braking in a straight line, which makes me think it could maybe be a sticking brake caliper, plus a very slightly warped disc.  Anyway, I'm going to book it in to a Hyundai dealer for assessment.

No problem, thanks.

I think you are on the right track.

 From your once every revolution comment, I would be looking for a high spot on one of the discs. Usually you can feel a bit of pulsing through the pedal, .

If its a high spot, usually  it is a discoloured area.

Could be binding in the rear calipers, on that model they are affected by European road salt and ice.
Given the potential rust you may have lost a damper spring beside a pad.