This is a strange problem, but we will do our best to figure it out.
The first thing to do is to apply the brakes very lightly when you hear the sound. You only need to bring the pads into contact with the disc. If the sound goes away when you do this, it means that the pads don't fit into the calllipers properly, and that normal vibrations in the vehicle are causing them to "bounce" around in the callipers. This means that you have to have the brakes inspected to see if all the anti-rattle plates are in place, or that nothing is obviously out of place.
It is likely that the ABS system is registering this as a fault, which is why the light comes on. However, you need to have a proper OBD II diagnostic check performed to see if there is not some other problem that could cause the ABS, or other system to activate the brakes at the speeds you mention.
It could also happen that the ABS is deactivating itself due to a problem it is registering, which is why the light stays on until you turn off the engine. The best advice I can offer you with the amount of information available is to have a diagnostic check done before you do anything such as replacing pads and discs. You may not need to do this, since the problem may not be with the discs and pads- it could be in the ABS (or other) systems.