The OP says 'when PC is idle' but we have no way of knowing how redleg is determining that. First step is to be positive very little is going on because this points to a different solution vs a load that they are unaware of. It's not uncommon for fans to spin up without that much going on overall (prolonged 10% overall CPU usage for example), esp. if you've taken no steps towards fan management of your own.
For the record, I love this FREE (fyi: I have nothing to do with it, this is not a commercial plug) software: https://getfancontrol.com/. It's not beginner level stuff and is very powerful but getting some basic fan control curves (speed vs temperature) out of it is not that hard. I would never be without this software on a PC of mine. One cool feature of it is it can put values of various temp sensors onto your taskbar. I have CPU and GPU temps available at all times. Having fan(s) speeding up w/o knowing temp readings leaves you in the dark as to why it's happening.
Another thing that is helpful to know in order to help is which fan(s) are spinning up? My PC (which I built) has like 9 total fans in operation, but almost any desktop PC has a fan for the CPU, and at least one case fan. Is it one of them, or both?
Given the totality of the description, first thing I'd do is visit the PC (or motherboard if it's home-made) makers website and get/install the latest motherboard/chipset drivers available. That's easy and can't hurt anything. Maybe it fixes the issue. Beyond that, we need more details to troubleshoot, redleg 
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