I have a 2005 Mazda Tribute, front wheel drive, 4 cylinder. When I accelerate I here a gear/whining sound. It goes away when I take my foot off the gas. It also goes away if I put it into neutral while driving. It started out like humming, but now is gear/whining. I have full acceleration though. Doesn't feel sluggish. Last year they rebuilt my differential but I don't know if they lied.

Hi!!
Take vehicle to Jiffy Lube or similar oil change place and have them change the differential oil, ask them to check for metal shavings. Also ask to have an additive or add the type of fluid that will help to reduce or remove the noise.
Good Luck!!

Sound like a Torque Converter got o bad.

It could be the alternator, AC compressor, power steering pump or serpentine belt ilder tensioner pulley. Mostly we have been finding alternators whining. Compressors and water pumps usually grind or growl. if the noise only happens when the car is moving it might actually be a bit of gear whine. if it makes the noise all the time it could probably be belt noise.
You'll need someone reving it slightly, to make the noise appear, while you localize it. Once localized, cost can then be estimated.

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There should be a decent warranty on a rebuild - you might want to check on the warranty.. It could also be the trani.. Even though automatic trani is hydraulic - it can make some pretty awful sounds..

Good luck ;)