So, lately I've noticed that somewhere around the front left tire, in the engine area, there is a buzzing/rattling. It starts around 18 mph and quits after about 20 mph. It happens when I'm high in second gear, and about to drop into third. 08 V6 auto. What could the problem be. I know its not the tire. Thanks.

 

Is it road speed related only and not engine speed? Does it happen with the engine hot and cold or just one or the other? What happens when you apply the brakes when it's buzzing? Turn on/off accessories?

 

I also have a buzzing/rattling vibration noise under the car. I am wondering if it is the exhaust heat shield or something. I have tightened up the exhaust, exhaust hanger clamps and other bolts around the area, but it is still making the noise.

 

Could be heat shield, the tranny separater plate or something along the exhaust. After I installed my axlebacks, I had a buzz at about 1800 RPM. I torqued down the axleback-to-midpipe clamps about 1/4-1/2 turn and it was gone.

 

I had that just like the OP said.
Buzzzz from 18 to 22MPH.
It turned out to be the PVC valve
going to the air intake tube...