Static crackle sound from new iMac

Hello. I recently bought a iMac (Retina 5K, 27-inch, Mid 2015) with a SSD drive. I love it, but I've noticed a faint static crackle sound (like a tiny geiger counter) coming from the back, around the power socket. It's just loud enough to be annoying, especially having spent so much on it. The problem is it's intermittent and I worry about taking it back (it's a big hassle involving train journeys) and getting the default, 'couldn't replicate the problem' reply only to have to bring it home again and be saddled with the problem.

I suspect it's the PSU. The sound doesn't alter or change if I change brightness or anything like that. My question is, why is this intermittent? This is what I haven't been able to find in other postings. Why is it fine some days and others it will happen regularly -- if it is the PSU? And will it get worse or do any harm to the machine?

Any thoughts would be welcome. Thanks in advance.