1996 Ford Crown Victoria • 100,000 miles

After I'm done accelerating and I've reached cruising speed, whether it be around 40-45MPH in town or at highway speed, I frequently get a low rumble for a moment (maybe 2-5 seconds). Modulating the accelerator seems to make it stop, and even if I don't do that it always stops on its own. I think I can feel the transmission doing something when it starts and stops (possibly the overdrive engaging/disengaging?).
July 4, 2012.

What you're describing is exactly the same as what can happen on some Chrysler models when the wrong transmission fluid is used. You will get a shudder when the lockup torque converter engages. It engages relatively slowly to cushion it for comfort. Fluid pressure to apply it builds up slowly and some slippage is designed in until it is fully engaged. Additives in the transmission fluid allow that slippage to occur smoothly.

To verify if this is the cause, hold the gas pedal steady at cruising speed, then lightly tap the brake pedal with your left foot. That should cause the torque converter to unlock and engine speed will go up a little. It will relock a few seconds later. If you feel the same shudder, suspect the torque converter or transmission fluid.

Jul 4, 2012.
I couldn't seem to _make it_ shudder by tapping the brake as you described above. But when it did shudder on its own, tapping the brake made it stop immediately.

Does this still point to the transmission or torque converter?

Jul 4, 2012.
It would seem so. If it's due for a fluid and filter change, that might take care of it. I don't want to tell you to ignore it in case it is going to lead to further damage, but in the case of the Chrysler products that do this with the wrong fluid, it's an annoyance that most people just live with and put up with until they get the fluid changed. Most mechanics are aware of the need to put the right fluid in and the complaints are more prevalent after a do-it-yourselfer changes the fluid and doesn't know special fluid is needed.

If my diagnosis is right, I suspect you should be able to vary the characteristics of the shudder by changing how hard you're pressing the gas pedal or how hard you're accelerating. Transmission temperature might affect it too meaning it might act differently when the engine is still cold compared to when it's hot from driving at highway speeds for a while.

If the shudder gets worse or lasts longer, then you might think about a leaking internal seal that is limiting how much fluid pressure is applying the clutch in the torque converter. My reasoning is this is not a common complaint on Fords so something more than just the wrong fluid might be going on.

Jul 5, 2012.