* Motor Mounts: Worn motor mounts allow excessive engine movement. When the engine shifts, it can put stress on the transmission and cause a clunk as the drivetrain settles or engages. This is more likely to be noticeable when the engine is warm because the engine is at operating temperature and may move more freely. Check for cracked or visibly sagging motor mounts.
* Torque Converter: While you've replaced the transmission, the torque converter may still be the original. A failing torque converter can cause clunking or shuddering, especially when the transmission fluid is warm. This is less likely if you replaced the torque converter along with the transmission, but it's worth considering if it was reused or not specifically replaced.
* Transmission Mount: Similar to motor mounts, a worn transmission mount allows excess movement of the transmission, leading to a clunk when shifting. Inspect this mount for cracks or excessive play.
* Driveshaft: A worn universal joint (U-joint) in the driveshaft can produce a clunking sound, particularly when the transmission shifts. This would be felt more when driving, but it's possible to feel a clunk when shifting into gear.
* Exhaust System: Though less likely, a loose or contacting exhaust component could be causing the noise. Check for any areas where the exhaust system might be touching other parts of the car.
Troubleshooting Steps:
1. Visual Inspection: Start by carefully inspecting the motor mounts, transmission mount, driveshaft, and exhaust system. Look for cracks, excessive play, or any signs of damage.
2. Test Drive and Listen: Have someone listen carefully to the clunk while you shift. Try to pinpoint the location of the sound (engine, transmission, rear end). This can help diagnose the source.
3. Professional Inspection: If you're not comfortable performing these inspections yourself, a qualified mechanic should be able to diagnose the problem more accurately. Provide them with the history of the transmission replacement. They can perform a thorough inspection, including checking the torque converter (if it wasn't replaced) and all other related components.
It's crucial to address this issue. Continued operation with a clunking transmission could lead to more significant and expensive damage. The fact that it only occurs when warm points towards something related to thermal expansion and movement, hence the focus on mounts.
