Occasionally, when listening to my system (through speakers or headphones) I get a short burst of a humming noise.
How do I find out what the source of this noise is, and more importantly, how do I get rid of it?

I will be checking to see if it is source dependent later today, as I noticed it yesterday when listening to vinyl.

At a guess, I would say it is probably in the 50Hz range, but it could be a multiple of 50Hz. It usually lasts for about 2 or 3 seconds.

I am thinking of fitting a filtered IEC to my mains block as a first step, as I think it is noise on the mains. Any other suggestions?