So I thought my 6-month-old hard drive was already going bad. I heard sounded like bad bearings in the platters so I reluctantly ordered a replacement drive, took out the old one, popped in the new one, started up and ... AAH! The noise was still there. Turns out it wasn't the drive but the fan. D'oh!
Since I now had a spare (apparently good) drive now, I decided to kill two birds with one stone so I also ordered a Data Doubler kit from OWC so I could replace the optical drive with the hard drive and have 1.5TB of storage to make the ULTIMATE MacBook Pro. Okay, maybe not.
Anyway, the repair went fine. Three screws, zip, zip, zip, out it comes, in goes the new one, put the Data Doubler in, put it all back together, plug it in, start it up... and nothing. Happily I immediately remembered that I forgot to reconnect the battery. Zip, zip, take off the back, connect it and all is good. Back to a nice quiet machine.
First, make sure the part you're replacing is the actual part.
Second, if you're going to be opening up the machine anyway, think about other repairs or upgrades you might want to do at the same time.
Third, diagnosis is very important. I wish I'd been able to tell the difference between a failing hard drive and a failing fan, but not sure how I could have avoided this.
Fourth, make lemonade from lemons. The mistake turned out to be a boon of additional internal storage.