Possible causes of knocking noise when changing speed in an automatic transmission:
Mechanical problems:
* Worn or damaged friction discs
* Damaged planetary gears
* Worn or damaged bearings
* Torque converter problems
* Low automatic transmission fluid level
Electronics problems:
* Faulty solenoids
* Errors in the automatic transmission control unit
Other reasons:
* Contaminated or aged automatic transmission fluid
* Worn or damaged automatic transmission filter
* Problems with the driveshaft
* Faulty engine flywheel
Possible solutions:
Mechanical problems:
* Replacement of worn or damaged parts (friction discs, gears, bearings)
* Repair or replacement of torque converter
Electronics problems:
* Replacement of faulty solenoids
* Reprogramming the automatic transmission control unit
Other reasons:
* Replacing automatic transmission fluid and filter
* Repair or replacement of the driveshaft
* Repair or replacement of engine flywheel
Diagnosis and elimination:
*Connect a diagnostic scanner to read trouble codes.
* Check the level and condition of the automatic transmission fluid.
* Inspect the transmission for visible damage or leaks.
* Test the transmission in various modes to determine the source of the knocking noise.
* Perform more thorough disassembly and inspection of the transmission if required.