I've been reading about your issue, specifically, a rattle that appears to be located in the vicinity of the pump unit/CPU. You have changed out the Khuler four times, and yet the rattle still exists!
A few questions...
Q1: Have you had this rattle happen on different systems that use the Khuler, or have all four Khulers been installed only in one (the same) system?
Q2: Have you gone around inside the case while the pump is mounted and running, and tried pressing down on various components and areas around the pump to see if they have any affect on the rattling noise?
Q3: Have you tried re-mounting the (rattling) Khuler in a different case on a different mobo, to see if it rattles in that install?
I will assume for a minute that the answers to 1 and 3 above is No; that all have been installed in the same case and mobo, and not re-tried in a different host PC before returning the units.
While I have little doubt that you have been both diligent and thorough in your efforts to diagnose your 'rattle', I must say that your receiving a fourth
bad unit in a row, is extremely unlikely and so far off the charts statistically-speaking that it really points to some other problem
in your system.
I suggest the problem is not defective units(!) from Antec, but rather is a problem with your CPU, motherboard, or case.
Certainly, these pumps produce at least a little vibration. But the majority of Khuler owners - by far, (and myself included) - have complained little or none about the chronic rattle you have. What I hypothesize is that the pump vibration is rattling the CPU itself, or the CPU chip ZIF socket on the motherboard, or components adjacent to the CPU, such as the VRM modules, the chokes, or the caps. Or, it is vibrating the whole motherboard and perhaps the khuler back-plate is touching the motherboard tray and rattling there, OR one or more mobo standoffs are loose on the motherboard tray (check for these!). It's even possible that your mobo has some sort of internal de-lamination between board layers and that is amplifying the vibration from the pump.
Take a few steps back and re-assess the situation. Assume the Khuler isn't bad; rather that, there is something about how the Khuler, the CPU, the mobo, or a component on the mobo is mounted such that they are vibrating and/or rattling against something. While you may have been pressing on components and areas around the pump to try to isolate the noise, I'm convinced something is not right in that area and more pressure may be needed or something has been overlooked.
Four bad units in a row just does not happen
, unless the units are being drop-kicked (every time!) by UPS, or by your landlord/wife/roommate, or you
are somehow (inadvertently) mishandling the unit each time you install it.
But I think it is not you or UPS at fault, I think it is a defective mobo or physical mounting issue (a badly mounted or defective CPU ZIF socket, or a bad choke, or a bad NB/SB heatpipe-heatsink-mounting-cooling fan-whatever). I urge you to look for something like that, extremely closely, before suspecting the (fourth!) cooler of being defective...meanwhile, you have my sympathy.
NB: One last idea - did you use the foam tape to mount the backplate? If not, that's another suspect - the backplate might be vibrating against the mobo).