What's this noise coming from?

My Outback makes a thunking noise when I let the gas or clutch out quickly. I took it to my mechanic, who replaced a broken transmission mount, but it is still making the noise. He works out of his garage and doesn’t have diagnostic tools or a lift, so has some difficulty with diagnosing problems. Otherwise he’s a very competent mechanic–retired airplane mechanic actually–so I trust his judgment. For the moment, however, he is stumped as to why the thunk remains. Any suggestions?

What year and mileage on your Outback?

I expect you have another loose or broken engine/xmission mount somewhere. Any acceleration or deceleration – from movements with the accelerator pedal or clutch – causes an inertia force on the engine which twists it and xmission in their mounts. The mounts are designed to allow a certain specified amount of movement, but no more. Something is probably moving more than it is supposed to, resulting in a misalignment of parts, such as the clutch linkage or the xmission to drive shaft linkage, and causing the clunk.

If it is not that, then the next thing, you’d have to look at all the stuff from the xmission to the drive wheels for something loose, broken, or worn out. I’m not familiar w/the configuration of Outback, but if your car has U-joints in the drive shaft, and since you already had a broken xmission mount, those U-joints would be prime suspects.


airplanes are not cars

and vice versa

I wouldn’t expect your mechanic to be an expert on cars

And I also wouldn’t expect a car mechanic to be an expert on airplanes

At least he’s not guessing. There is nothing wrong with replacing a broken mount. The first step is to replace obviously broken parts.

Is the noise coming from the back of the car? If so, you may have what we Subaru owners call a “thunk in the trunk.” What’s happening is the differential is hitting the bottom of the trunk from the accelerations (actually, the change in acceleration, which is called jerk or jounce) when you change gears or do something that will generally upset the drivetrain. If this is happening, the bushings that attach the differential to the rear subframe are shot. They weren’t that good from the factory to begin with. So have your mechanic check out the rear differential mounting points. He was on the right track with the broken transmission mount, so I wouldn’t doubt his competence either.