there's a loud knocking or clanking noise coming from the hammer mechanism, not the normal pounding sound.

Diagnosis

The loud knocking or metallic clanking noise in the Bosch GBH2-28L indicates a failure in the percussion mechanism, specifically a breakdown of the striker assembly or the cylinder wall. Unlike the rhythmic, muffled thud of a healthy rotary hammer, a clanking sound suggests that the striker (the weighted steel piston) is no longer traveling in a linear path or is impacting the cylinder wall due to excessive play. This is typically caused by the wearing down of the internal lubrication grease or a physical fracture in the striker head. When the striker develops a hairline crack or the retaining ring slips, the component shifts off-axis. This creates a "metal-on-metal" slap every time the tool cycles. You can confirm this by running the tool in "hammer-only" mode at low speed; if the noise is erratic and sounds like a loose bolt rattling inside a pipe, the striker has likely failed. If the noise is accompanied by a significant drop in drilling power, the piston is likely leaking air or failing to make a full impact against the drill bit shank.

How to Fix It

This repair requires a complete teardown of the hammer mechanism. Ensure the tool is unplugged from the AC power source before beginning.

1. Remove the four screws from the outer plastic housing and separate the two halves of the shell to expose the internal motor and gear assembly.
2. Unscrew the gear housing bolts and carefully slide the gear assembly away from the hammer mechanism. Be careful not to lose the small shims located behind the gear housing.
3. Locate the hammer cylinder. Gently tip the cylinder out of the chassis. If it feels stuck, do not force it; check for debris or hardened grease around the seal.
4. Extract the striker (the free-moving steel piece inside the cylinder). Inspect the striker for pitting, flat spots, or cracks. If the striker is chipped or the retaining ring is deformed, replace it with part number 1 607 001 006.
5. Clean the interior of the cylinder using a lint-free cloth and a mild solvent to remove old, contaminated grease. Inspect the cylinder walls for scoring or deep scratches.
6. Apply a fresh coat of high-temperature molybdenum disulfide grease to the new striker and the inner walls of the cylinder. Do not overfill, as this can cause hydraulic locking and reduce the impact speed.
7. Reassemble the components in reverse order, ensuring the gear housing is seated flush against the motor housing before tightening the bolts to 4 Nm of torque.

If That Didn't Work

• Check the Chuck Retainer: Inspect the SDS-plus chuck for excessive wobble. If the internal locking ball or the spring-loaded retainer is worn, the bit will shift during impact, creating a clanking sound that mimics a failing striker. Replace the chuck assembly if the bit has more than 1mm of lateral play.
• Inspect the Gearbox: Open the gear housing and check the planetary gears for missing teeth. A chipped gear tooth will create a rhythmic "click" or "clank" that synchronizes with the motor RPM rather than the hammer impact. Replace the gear set if any tooth is sheared.
• Verify the O-Rings: Check the rubber O-rings on the piston assembly. If the rings are torn or brittle, the air compression is lost, causing the striker to "slap" inconsistently against the piston. Replace all seals in the percussion group.
• Check the Motor Commutator: If the sound is an electrical "snap" rather than a mechanical clank, the motor brushes may be sparking excessively or the commutator may be pitted, creating an acoustic vibration that sounds like mechanical knocking.

Heads Up

• Never use the tool in "rotation-only" mode for extended periods when drilling into hard masonry; this puts undue stress on the striker and can lead to the exact failure described above.
• Avoid using low-quality, generic SDS-plus bits. Bits with improper tolerances can cause the striker to hit the piston unevenly, accelerating wear on the cylinder walls.
• Periodically blow out the ventilation slots with compressed air. Dust buildup inside the housing can overheat the grease, causing it to thin out and leave the striker unprotected.
• Always ensure the bit is fully seated and "clicked" into the locking position before triggering the hammer function to prevent shock-loading the internal mechanism.