Are you sure that the noise is not a fan ? Since it only happens under heavy load then it could be the cpu heatsink fan. I would take the side panel off to see if you can get a better feel for where the sound is coming from. Are there any indications that the drives performance is being affected by this or it's heating up or anything besides the noise so that you can maybe contact WD support and possibly return it for a new one.
My Hard Drive runs at a perfectly stable temperature. I just opened the case and ran the stress test again and found out that my Drive bays were what was rattling. Thanks for replying and helping me fix my problem man!