A click or buzz noise that happens every 10 or 15 minutes, could be coming from another electrical device on the same mains wiring, for instance, a fridge, or central heating thermostat.
Each time a fridge motor starts up, it puts a 'spike' of noise into the local mains wiring, which is amplified by items such as Hi-Fi or PC speakers.
If that is the case, it is possible to buy for a few pounds (dollars), a small choke that will clip over the mains lead, near the plug, which will cut the interference down.
Also, there are mains extension cables with multiple outlets designed for HI-Fi of PC use, which have spike suppression built in. It is sometimes called 'mains conditioning'.
These are not the same thing as £5 extension 'gangs' !
The prices vary wildly, but here is one I found quickly.
http://www.saverstore.com/product/2...-Mains-Extension-Lead-with-Power-Conditioning
Or try (if in UK)
http://www.maplin.co.uk/module.aspx?tabid=1&criteria=usb&moduleno=219077&c=so&u=strat15
Also: From past experience with interference in a HIFi system, the problem turned out to be coming from the central heating boiler next door! In that case the fault has to be stopped at source. You might try fitting an RF filter to your speaker or headphone lead. I have a similar 'Choke' on my PC speakers. I bought a DAB radio this year, and a similar device is fitted to the mains adapter.
http://www.maplin.co.uk/Module.aspx?ModuleNo=32799
These clip on ferrites are designed to go over the mains lead to prevent the interefence from running up the lead, but if you use headphones for transcribing work, or audio typing, it might be better to wind the headphone lead through this choke, several times over.