CyberPower CP900EPFCLCD Input: 220-240V~, 10A, 50/60 Hz UPS Output: 220-240V~, 4A, 50/60 Hz, max 900VA / 540 WOn this picture you can see the internal battery.
Horrible sounds! As soon as the internal fan was running, or trying to run, rattling noises could be heard. When moving the device, the noise would change, and when pushing the sides of the box slightly inward, the fan could be heard coming to a stop.
What to do, RMA or repair myself? I decided for the last option, mainly because I don't really trust the repair services of the vendor (typically they just ignore any problems you report, unless you send in a device more than once -- that is my experience at least with other vendors). Also, looking for the problem myself would give me a better understanding of the fire risks, if any.
Here is what I found: a 220 V cable (see picture, the big black cable running between the (yellow) transformer and the (small black) cooling fan. Not only was it very close to the cooling fan, it also had much slack so that it easily could touch the fan. This perfectly explains the noises I have been hearing. But .. this also means that if the problem would be left unsolved, in time the power cable could be loosing all its shielding, and this would indeed become a serious fire risk.
Solution: tie wrap!
I also removed the battery. Nope, I am not stupid enough to work on the internals of a UPS with the battery still attached. When reconnecting I found another safety issue. The connectors of the UPS (the ones with the wires attached) need to slide all the way onto the battery connectors. To enhance security, an additional shielding piece of plastic has been added to the red connector on the battery. See picture.
Because of this piece of plastic, it is not possible to slide the connector all the way to where it should be (there is a small 'bump' that you can feel and you need to push it past this bump). As a result the red (+) connector can come loose quite easily. I am not sure whether these devices are connected properly when they come from the factory.
Solution: utterly simple, remove the additional plastic cap.