imac purring noise

My iMac began purring yesterday. It's a late 2015, 21.5" iMac with a fusion drive, bought new in 2017, running 10.15.4. The noise is gentle, like a cat purring, or a very small woodpecker. It has a two second cycle before it repeats. It's difficult to locate where inside the iMac it's coming from, but it seems to be central. I attach a link to a youtube video of a recording: https://youtu.be/yhE4S5kNG4s. I've run various diagnostic tools and the hard drive appears to be fine, and as it's a fusion I presume it doesn't have major moving parts. The only potential anomaly is the internal temperature running a little high, so I'm guessing this noise is a faulty fan. I guess a repair isn't easy to get during lockdown, but of anyone thinks it's something else I need to be worried about, I would appreciate the heads-up.