TL:DR;
Lube up the fan.
I've experienced the same issue after about 6 months. At first, using compressed air through the bottom vents fixed the issue for a short while, but the noise returned bigger, badder and more persistent.
I decided to take the bottom off (screws under the rubber pads, a very common tech thing) and take a look inside. After a proper clean with the fan on and off, the noise persisted. It was on its last chance before I disconnected the fan entirely and considered a replacement fan.
- If you remove the fan, you can pry the blades off of the motor/frame assembly. It's literally just pressed in and clips in at the center rotation point. Once the blades are off, you can clean the motor and fan with an earbud and some solution, then apply some oil to the center part where the fan clips into the motor and rotates. Reassemble the fan and reinstall. Problem solved.
If you want a reason why the problem happens, I'd say that Zyxel cheaped out when choosing a fan. What is it about the fan that's cheap? I'd say that there's either too much of a gap between the blades and the motor/housing components, allowing dust to get into the pin/socket joint and making a less-than-smooth rotation. Or the oil used in manufacturing the fan has dried out (or similar, maybe too thin) and is no longer sufficiently lubricating the fan.
Either way, it's absolutely abysmal that EE don't have an adequate replacement/repair service for such a fix. They should be ashamed for how this is being dealt with and the treatment paying customers are receiving.
Obviously they've struck a deal with Zyxel, and are stuck with thousands/millions of units. But someone isn't taking responsibility, and it shouldn't be up to the customer to figure out how to fix their problem, just because it's cheaper to push forward and leave it for customer services to pass the buck until they come out with a new device, instead of publicly admitting a fault and affecting their bottom line.
May all their ice-cream taste like earwax, a hex upon thEE!!
