So long as you are happy that this clicking is not coming from your hard drive but is coming through your speakers, it is most likely to be pick up in the speaker amplifier. If you are using fairly standard type computer speakers, where the audio goes to one of them which has an amplifier built into to feed both sets of speakers, typically these are not very high quality amplifiers.
Most houses, and offices for that matter, have a considerable number of mains powered circuits which switch on and off at more or less random intervals - lights, fridges, freezers to name but three - and all of these can generate switching 'crashes' which echo around both by the wiring in the house and as RF energy. Filtering most of this out is not too difficult or expensive, filtering virtually all of it out is both difficult and expensive ! This is one of the reasons high quality amplifiers cost so much. Since you are happy that the problem is not your hard drive, then this is the probable cause of the clicks on your speakers.