Howl?
 

Hey guys, I have an 05 Mazda 3s in stick. When i back up, the car makes this howling noise. Now, in every stick car I've been, I've heard this noise upon backing up. Do you guys know what this noise is?

All MTX make that noise while going quickly in reverse. The ratio of the gear is lower than the ratio of first gear going forward. So the engine is working harder and the tailshaft is turning faster.

It's also related to the synchronization of the gears, much like ICSpots points out.... used to be that many an import with a manual trans made noises in a few forward gears, too... in order to get away from it, you had to be sure your car had (or you optioned up to it!) the "synchronized" transmissions...

Generally, its because the reverse gear in manual transmission cars is a straight cut gear instead of being helical cut.

For the most part, being sych'd vs unsych'd for a gearbox is just the difference between needing to double-clutch when shifting, and not. The noise would be from straight-cut gears (cheaper to make)