If we're talking about the tall-block coolers, then I think the problem was/is that the impeller's shaft is loose in its bearings and it literally rattles.
As for the flat Corsairs units, they have so many problems that I wouldn't know where to begin, but the rattling specifically is hard to explain and investigate, I believe it's either a mechanical problem with the sleeve bearing or a resonance in BLDC windings caused by the PWM signal and the frequency they chose for it. I could confirm that, but like I said, I don't have a unit to dissect, I designed the diode mod on a cooler that had to live and gathered the rest of the knowledge from my own research, supported by detailed photos of the unit's internals provided by a friend.
If you're interested in more details, you'll find them here:
starting from post #26.
If anyone was willing to donate an H60/H80/H100 for science, I would gladly apply the findings.
Not many volunteers, eh?