I own a 2004 chevy malibu classic and upon reaching 20mph a whining noise starts and changes in pitch as i increase in speed.. also occasionally my dashboard lights will dim in and out.. any advice on this matter will be greatly appreciated...

Can you tell where the noise is coming from?

appears to be coming from the left side of the engine near the alternator

Could it actually be the alternator? Those symptoms sound a bit consistent, with the sound coming from that area and the lights dimming. If you have an Autozone nearby they do free alternator testing and could at least tell you if there are issues with it's output.

Good call, #1. That would be the first thing I'd check out, too.

actually the alternator is brand new.. has been replaced.... twice.. but the problem persists through alternator change..

Did you check the Belt and Tensioner? If the Tensioner is weak, It will cause a charging problem.
Which engine is this?

I'm not sure on the belt routing and pulleys, but make sure to check the idler pulleys. They can whine when they are dying. I found this the hard way...

Another good suggestion. I misread the car- it's a 2004 Classic (aka - 97-03 'bu style). Our 97 not only had the alternator replaced twice, but the pulleys needed replaced later. I can't remember for sure, but the noise described here might've been one of the symptoms that caused me to take it in when the service department discovered them going bad.

You are lucky you can tell what's whining. I didn't find my idler pulley whining until it was too late, as my supercharger (Grand Prix) also whines really loudly. Shredded both belts and the plastic pulley was no more-only a bearing left.