Bicycle Mechanics - Knocking noise from the rear wheel when braking

I keep getting a light knocking noise from my rear wheel only when I apply the brakes; there's no issue when the wheel is rolling and the front wheel is fine. I checked the brake pads thinking there might be something stuck but they're in good shape. They're Ksyrium SL wheels and are fairly new so I'm not quite sure what the problem might be. Any ideas?

This has happened to me and others when the wheel needs truing. Check the wheel and tension/true if needed.

roadiejorge, Check the rim at the seam as it may not be flush, you may also see a build up of pad material. You may need to file the high side down to be rid of any step or replace the rim.

Brad

Or it could be a gouge in the rim. Just hold it between your fingers and turn it to find the spot.

I took it to the bike shop last week and they fixed it but two rides later it came back again.

I took it to the bike shop last week and they fixed it but two rides later it came back again.

Take it back to the same shop and ask them explain what is happening.

Take it back to the same shop and ask them explain what is happening.

I intend to once their head mechanic gets back from vacation. I swapped out the Ksyrium SL wheel that's giving me the problem with the Ksyrium Elite wheel on my other bike and it worked fine, and interestingly enough the SL wheel worked just fine on my other bike. I'm at a loss as to what would be causing this only on one bike and not the other, any defects on the braking surface should have been detected on the other bike.