If it's shifting normally, you might be hearing some shift solenoids clicking rapidly to soften the engagement into different gears so the shifts aren't so harsh. That is normal, but if you haven't heard that before, you might look for a missing sound shield, usually made from foam rubber. The clue I'm going by is your observation that it's coming from the driver's side. An additional clue to look for is that it will occur for usually not more than one or two seconds at a time, and you'll hear it when you shift into drive or reverse.
If the noise doesn't seem to coincide with the transmission shifting into any one gear, figure out if you ever hear the noise while in park. That would allow you to stand under the hood and listen for the location.
There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.
Saturday, April 23rd, 2011 AT 5:54 AM