The noise you describe is often caused by a defect in the tire. It probably made the same noise before it was rotated but was in the back and therefore you didn't notice it. I'd bet if you spin it you will see a lateral sway or bump while it turns if you watch the tread. This is caused by a belt inside the tire breaking or shifting.
Most tires carry a warranty which covers this though the last set of Pirelli's I had they claimed it was caused by road hazard (pothole) and therefore only the selling dealer's road hazard warranty would cover it which they kindly did and replaced it. Still, I didn't like the Pirellis and went for Michelins last year and have been happy with them.
If you can't see the problem by the method I describe above, just mount the spare on one front side and drive, if it went away then it was the one you removed. If not, then it is the one which remains. Then you can go back to where you bought the tires and have them examine it more closely. You can balance a tire with a bubble or shifted belt, but it will still make that noise.
Also, you didn't say clearly where the worn tire WAS while it was wearing but if it was on the right rear then I can tell you I have noticed the exact same thing on my W210 and others too have said this. If it was on that corner the whole 18K miles it will wear much faster on the right side than on the left side, it has to do with the open differential. Now I rotate mine every 5K like clockwork to avoid that. See my old post here and the explanation for it at the end in post #6 and #7:
W210 1998 E300 Right Rear Tire Prematurely Wore Out