Oldsmobile/Buick Repair: car shaking and making noise, prix auto, ac compressor

Question
Hey Chuck,                                                   My name is Jorge and I am having a problem with 2000 Gran Prix auto, 3.8 liter, not supercharged. First it started as a noise when accellerating but now it is shaking when I am driving and the car is changing gears. The light came on when the car started to shake. At first I tought the noise was coming from the ac compressor cause it started when I turn it on full blast one day. I changed it and problem pursisted. I had my brother check the vehicle and he thinks it is the belt tensioner cause he put presure on the belt and the arm on the tensioner had about a half to an inch of slack. Thanks for any help you can give me.

Answer
I almost bet you have a misfire, the light is a po300 code (multiple misfire). At first the a/c put on a load making it worse, but now it's gotten worse. If you are mechanically inclined Try tuning it up, plugs,wires,filters. It also wouldn't hurt to see, on a scanner which cylinder is the problem.