noisy samsung rs21dgrs?

I've read lots about people having problems with the Samsung RS21DGRS defrosting problem, so before I start - we upgraded the fridge a year ago with the improved defrosting components/solution part of the fridge freezer and it has been working perfectly ever since.
So to get to the problem; Recently our Samsung RS21DGRS has become noisy - I've located the source of the noise (which only starts when you shut the fridge door!) from the lower rear printed circuit board (ET-R600a Ver 1.2) next to the water valve. I also felt each of the relays and I'm quite certain that the noise is coming from one of the relays on the PCB - I assume it's the fridge lamp relay as it's labelled (RLAMP). When the fridge door opens the noise stops and when the fridge door microswitch is closed (open circuit) the noise starts and the lights in the fridge dim and start to flicker at high frequency!
I removed the microswitch in the fridge and the noise immediately starts. I continuity checked the microswitch and it was ok for both open and closed circuit. I measured the resitance of the 2 wires at the fridge microswitch and it's 16.75 ohms.
The front panel is working perfectly and when I hold the top right and left buttons in for 10s no fault code comes up on the panel.
Finally, a repair guy came and changed out the fridge door microswitch (no change!) and the ET-R600 board at the back of the fridge, no change. Fault remains.
Any ideas?