* Torque Converter: This is the most probable cause. The torque converter acts as a fluid coupling between the engine and transmission. If it's failing (worn clutches, internal damage), it might not properly transfer power smoothly when under load. A failing torque converter can cause knocking, shuddering, and poor performance when the transmission is engaged.
* Transmission Problems: Several transmission issues could cause this:
* Low Transmission Fluid: Check the transmission fluid level *while the engine is running* and warm. Low fluid can cause all sorts of problems, including knocking and poor performance. Also check the fluid's condition – it should be reddish-pink and clear, not brown or burnt-smelling.
* Worn Transmission Components: Worn clutches, bands, or other internal parts in the automatic transmission can cause knocking and poor performance under load.
* Transmission Mount: A worn or broken transmission mount can cause vibrations that are felt as knocking.
* U-Joints (Universal Joints): These connect the driveshaft to the transmission and the rear axle. Worn U-joints will clunk and knock, especially under load.
* Driveshaft Problems: A bent or damaged driveshaft can also cause knocking and vibration.
* Rear Differential Problems: Issues within the rear differential, such as worn gears or bearings, can manifest as knocking and poor performance.
Troubleshooting Steps:
1. Check Transmission Fluid: This is the absolute first step. Low or dirty fluid is a common cause and easily checked.
2. Listen Carefully: Try to pinpoint the location of the knocking. Is it coming from the front (torque converter/transmission), the middle (driveshaft), or the rear (differential)?
3. Professional Inspection: Given the complexity of potential issues, a professional mechanic is best equipped to diagnose the problem accurately. A proper inspection might include a road test, transmission fluid analysis, and potentially dropping the transmission pan to inspect for wear.
Ignoring this problem could lead to more significant and costly repairs down the line. Don't continue driving the vehicle extensively until you get it properly diagnosed.
