What's the thud/Clunk sound in my drivetrain when in overdrive?
 

I have an 04 Mustang GT auto and when I'm cruising softly around 50-55 mph and stomp on the gas pedal, I hear this clunk sound. It doesn't happen at all speeds, but when it does, it is very repeatable. My best layman description would be as if there was some slack in the driveshaft.

That's not good, is it?

Could be U-joints. But seeing as it is a new car I wouldn't think that would be the problem.........but it is a Mustang GT! }> :9

Put your parking brake on and your car in neutral. Crawl under your car and grab the drivedshaft and turn it back and forth. Watch your ujoints and see if there is any play in it. It could also be your differential.

Good luck.........

-Matt :-tap

It feels like it is shifting really hard. Like you would popping the clutch on a manual. It only does it around 45-50 mph when it's in overdrive and has to downshift to 3rd. At speeds above 50, it just accelerates. At speeds below 45, is shifts smoothly into 3rd and takes off like a bat out of hell. It's great, except for that 45-50 mph range.

Thanks for the inputs. I'd rather go to the dealer service center with a little knowledge of what it might be than show up totally clueless.