Going to bump this thread again with a question:
Automatic transmissions eventually lock the torque converter up so that it is directly driving the drive train as opposed to the fluid acting as the coupling, for maximum efficiency, right? What are the general conditions that need to be met to lock/unlock it?
The reason I ask is because I'm wondering if this "clunk/thud" I'm hearing has something to do with a sudden release in stress in the drive train. So, for example, let's say I have some worn driveshaft components. As I am slowing down, if the torque converter is in this lock up mode and then suddenly releases while the engine is being driven by the wheels, that sudden release on the drive train could conceivably mean those parts under stress would sort of "lunge"/clunk into place as they find their normal equilibrium. Since this "clunk" I am hearing is always at the exact same speed when I am slowing down and the trans shifts into 3rd, I'm theorizing that this is also the point at which the computer tells the torque converter to unlock (provided it was locked up in the first place).
There is a lot of "ifs" in this theory....does it seem sound?