Sorry this is a bit off topic, apart from it's the audio amp used with the television and Sat receiver. I couldn't think of where else and it's a strange problem someone here may have an idea about?

Woke up about 4am the other morning to what sounded like a truck outside. After a long time, got up to have a look and as soon as i put the light on, it stopped.

Then last night it happened again... but I traced it to the hifi where a loud hum and buzz could be heard, from the speakers, surely?  I leave the amp on all the time.  Again, turned on a light - and it stopped.

Any thoughts?