Today I looked at Xbox #2, which made some strange fan-based noises. I checked the fan, and in fact, the CPU fan (which made the noises) had a much larger axial tolerance than the GPU fan. With some minimal force I could even detach the fan blades! It seemed that the clamped connection wasn’t in place, and touched a non-moving part. I simply removed the clamp, so the fan is now only hold in place via magnetism. But that should be enough, hopefully. If it fails again, I will completely replace the fan with another one.
The second issue the box had was that it read DVDs very poorly. Again, all of my boxes had the same DVD-ROM (Hitachi, ROM VER. 0047DH), but this particular box refused to read my Linux disc. I fixed that by tweaking some trimpot on the DVD pickup (from 3.00kΩ to about 2.59kΩ, if anyone cares). Now it reads the disc fine!
Again, what worries me again is that this particular box had two independend components out-of-specs.