One simple test first: When the car is cold and with AC turned OFF so the electric fans aren't running does the sound stop?
If so turn the AC back on and check if the noise starts. If it did, then go back and turn off AC again to verify that it really is one/both electric fan(s) that causes the sound.
Otherwise: If it's a big/difficult job depends on your skills, tools and place to work at. My best advice is to buy a good service manual, I like the one Bentley published - it's reasonable for DIY jobs. Do note that it is written for US spec. cars, how much is different for RHD (moved or mirrored) I have no idea.
Hunting down noises on an engine can be tricky, it takes experience to be good at it (which I don't have). What you also need is understanding of how things in the car works (thus how they should sound), a huge load of patience and methodically work.
Some noises can go for a long time before the component fails others not. And in some occasions the sound gets worse before failing other times that isn't noticeable. So when you have unusual noise be prepared something can fail and in our engine a fail might result in an engine rebuild. I don't write this to say you shouldn't try to fix it yourself but so you are aware of the consequences of not fixing it correctly and in time.
While I have worked at a garage I'm not a trained mechanic, I might be able to locate the fault but there is no way that I'm able to write a guide on how to troubleshoot it nor would I take it on as a job on anyone else car.
Another method trial and error, replace thing on a whim (oh someone said it's probably that part lets change it). Nope wasn't that, NEXT! It's an expensive and inefficient method but there are a surprisingly large amount of people using it, including not so skilled mechanics. Sometimes even the most experienced guy is forced to do this since troubleshooting just gets to expensive so replacing a common cause makes sense.
What a good mechanic will do is to spend up to an agreed amount of time troubleshooting and see what he can find / exclude. Lets say something like two to three hours if he gets to work undisturbed. And you want a skilled mechanic (authorized or not doesn't matter in my book) , avoid the cheapest garages - that would likely be wasted money.
