Odd noise from iMac late 2013

[karnsculpture]

Last week or so my iMac has made an odd noise occasionally, a single electronic note / noise like an extended beep that starts quietly and gets louder until it cuts off maybe 2 or 3 seconds later. I have checked things out as best I can but everything appears to be working - what could it be?

 

[elf69]

Is it possible to upload a sound file or video to help diagnose the problem?

Does the sound come from the internal speaker?
Could it be the PSU with a failing component maybe?

 

[karnsculpture]

Is it possible to upload a sound file or video to help diagnose the problem?

Does the sound come from the internal speaker?
Could it be the PSU with a failing component maybe?

Yes the sound comes from the external speaker. I can't recreate what causes it unfortunately.