Killing GSM Buzz with Ferrite Beads
The interference comes from GSM being a time division technology and the transmitter being switched on and off quickly. Poorly designed amplifiers can unintentionally receive this signal and convert it to an audible signal. The problem could also be in the signal source.

The principle behind ferrite beads removing the interference is that they would create a high impedance for the RF signal to get to the audio amplifier. The easy way to find out if your problem is with mains (or the amplifier internals in the speaker) or audio signal cables is to disconnect the audio input. If you still get the noise with only mains connected and speakers powered, the problem cannot be fixed with ferrites on the audio cable.

I won't repeat the use information from the wikipedia page so take a look at that if you didn't.

To purposefully induce the noise for testing your filtering, you can for example call yourself - it will signal the network even though the call will not likely be connected. Try moving the phone close to the audio signal cable, the speaker itself and the signal source to find out where your set up is most vulnerable.