When i'm on the gas you can't hear it but as soon as you let off a whining sound starts touch the gas its gone,its a 98 4 runner 3.4 engine 2 wheel drive auto,any help would be great

Might be the torque convertor. My tranny is going on my runner, and it has a whine right before it shifts and I'm positive it's the torque convertor. Might be in the reverse for you, when it becomes unloaded it starts to make noise.

Though it's only a theory...

Check your driveshaft as well. I had a similar sounding noise last year, took me forever to figure out. Lubed the driveshaft, no more noise.

4Runners.org - Lubing Your Driveshaft

Hopefully it's something simple like this.

Sounds like a bearing in your rear end.

From: Diagnosing Differential Problems | West Coast Differentials

Driveline, Axle, Ring and Pinion Gear and Differential Noise

Noises, vibrations and other irregular driving characteristics can often help diagnose a differential or driveline problem. Here are a few typical symptoms and their possible cause:

Whirring noise only while decelerating at any or all speeds is most likely caused by bad pinion bearings or loose pinion bearing preload, and almost never by bad ring and pinion gears.
A howl or whine during acceleration over a small or large speed range is usually caused by worn ring and pinion gears or improper gear set up.
Rumbling or whirring at speeds over about 20 mph can be caused by worn carrier bearings. The noise may change while turning.
Regular clunking every few feet may indicate broken ring or pinion gears.
Banging or clunking only on corners can be caused by broken spider gears, lack of sufficient positraction lubrication, or worn positraction clutches.
Rumble while turning may indicate bad wheel bearings.
A steady vibration that increases with the vehicle’s speed can be caused by worn u-joints or an out of balance driveshaft.
Clunking only when starting to move or getting on and off the gas might be loose yokes, bad u-joints or worn transfer case or transmission parts

After reading the reply I can't hear the noise when below 30 mph and it sounds like a mid range sizzling