iMac 3.5mm audio-out crackling and popping. Help!

Hello wise and wonderful Apple users!

I wonder if anyone can help. This has come on very suddenly. Yesterday: No issues. Today? No audio through headphones. First base covered: It's not the headphones, as I've tried several pairs. It's also not the internal speakers, as they work fine.

Symptoms:

Loud crackling/popping in left headphone, quiet or no audio in right headphone.

Tried:

Restart

PRAM zap restart

Fiddling with levels, output settings, LR balance

Changed Audio-Midi setup to 44100 Hz 2ch-20bit Int as per an earlier thread I found.

None of these have made any difference at all. Any advice gratefully received before I have to lug it back to the Apple Store once again!!

Cheers

iMac- 27", Mid 2011. 3.4Ghz i7, 8GB Mem, OS X 10.9.1