If it's well-used, it could just be the bearings are dry, it's a bit of a faff to do buy you can strip them down and re-grease the flat roller bearings, and oil the sleeve bearings, I did just that in my G3's TD unit and it quietened down dramatically, so it's worth doing if you have the time...
The Tech-Drives do make a sound. To me theyre all of the same. I see that new G5 you just got; I would compare the sound of it and the Sentria and see if theyres a difference.
I know the sound of the Tech Drive, but this one seems abnormally loud. I have compared it to all of my Generations and it sounds so much different. Since the innards of a Kirby are just to complicated for me at this age, I know that sounds sad, but its true, I'm going to take it to a friend of mine and have him service it.