Possible causes of knocking noise when braking with an automatic transmission:
Brake system:
* Worn brake pads
* Overheating of brake discs
* Defects in brake calipers
Suspension:
* Worn anti-roll bar bushings
* Defective ball joints
* Damaged shock absorbers
Transmission:
* Torque converter malfunction
* Worn transmission gears
* Differential defects
Drive shafts:
* Worn constant velocity joints (CV joints)
* Intermediate shaft malfunction
* Defects in the driveshaft (for rear-wheel drive vehicles)
Wheels:
* Unbalanced wheels
* Deformation of rims
Other reasons:
* Loose wheel nut
* Free rotating wheel hub
Diagnostics:
To accurately determine the cause of the knocking noise, you must perform diagnostics as follows:
* Inspect the brake system for worn pads, overheated rotors and defective calipers.
* Check the suspension for worn bushings, ball joints and shock absorbers.
* Diagnose the transmission by listening for abnormal noises and checking the transmission fluid level.
* Inspect the drive shafts for worn CV joints or other defects.
* Check wheel balancing and for any deformations.
* Tighten the wheel nuts to the proper torque.
Repair:
Depending on the cause of the knocking noise, the following repair work may be required:
* Replacement of brake pads or discs
* Repair or replacement of brake calipers
* Replacement of worn bushings or ball joints
* Replacing shock absorbers
* Repair or replacement of torque converter or gearbox
* Replacement of CV joints or drive shafts
* Wheel balancing or wheel rim replacement
* Tightening wheel nuts or repairing wheel hub