iMac Headphone interferance

Small thing, big annoyance.

My headphone socket went on my work iMac shortly after the warranty run out which meant I couldn't listen to music. Imagine 8 hours of listening to work drivel...
I combated this by getting a USB Sound card from eBay for a couple of quid.

Intermittently I get noise interference which is deafening to the max and I have no idea where it comes from. It's only just started happening after about a year, I've just bought a new USB sound card, tried it in different ports and it's still doing it.
I might have to leave my job if I can't sort this.
Any ideas people?

I've reset the ports and tried different headphones but it's still happening. It also doesn't happen to any other mac in my vicinity.
I'd love to know what this is.

EDIT: Wondering if it's a coincidence that the sound card went and now this is happening?

You say, "My headphone socket went on my work iMac shortly after the warranty run out".

What exactly do you mean by "went"? And what caused it to "went"?

What does the interference sound like i.e. mains hum, crackling, rustling?

It's possible that the jack socket may be broken and the output may be shorting. If this is the case, an internal investigation should reveal the problem.