Does it only happen while listening to the radio or does it also happen while listening to a CD?

If the wires on the speaker end are secured, the next step would be to check the connections at the amplifier. Make sure all the connections at the amplifier are secure and undamaged, especially the ground wire/connection. An improper ground can cause problems like the ones you are describing. If the connections on the amplifier are secured, the next step would be to check the location which the amplifier is grounded to the body. Again, check that it is secured, undamaged and without corrosion.

Next, you can check to see if the speakers are damaged, however, if all the speakers have the same problem, it is most likely not the speakers that are the root of the problem. The easiest way to check to see if the speakers are damaged is to grab a multimeter and check the resistance on the coils. Basically, set the multimeter setting to resistance or ohms (Ω), then put the probes on the corresponding wire. The stock speakers are 4 ohms I believe so a reading between 3.0-5.0 ohms is acceptable. If you get a very large reading(infinity) then you know the speaker is damaged.

If all of these check out, it is possible that there is an internal problem with the amplifier or there is something wrong with your head unit.