I bought my first Mazda two months ago. It's a Mazda 6. Since I have purchased it, it is making a whistling sound that seems to be coming from under the car. I took it in for repair and they replaced a belt and pully which did not fix the problem. Anyone have any ideas? I can hear it more when I go through a drive through fast food place or pull in my garage.

 

It sounds worse when you have a wall for the noise to echo from. My bet is it's a belt drive/ tensioner/ idler or alternator issue. Those belts are very narrow and are grooved to increase the load bearing area, and they do tend to make the engine sound like a turbojet on finals, especially when they get wet. There are a couple of things to watch for though; the first is they have to be carefully aligned, so the pulleys have to lie dead on in the same plane as the belt so there's no tendency to "ride up"; any skew and they will scream. The second thing is that they stretch, especially just after replacement, so the tension has to reset after a couple of months or they'll slip and heat up. I think the worst culprit in this respect is the alternator pulley which has a really small diameter.
Apart from the drive belt issue, there's the infamous electro-hydraulic steering pump saga. These things haven't got the bugs worked out yet, and they do make some mighty strange noises. You can check this out by seeing if the noise gets much worse when you put on full lock, BUT ONLY DO THIS FOR A SECOND OR TWO, or you risk joining the failed power steering pump club.