I think Ski nailed it. A mechanical gearbox like yours can be noisy under certain circumstances.
There is even a disclaimer in the latest ZF 25 A factory brochure:
Torsional Vibration and Torsional Couplings
The responsibility for ensuring torsional vibration compatibility rests with the overall propulsion system integration responsible party. Compatibility check of torsional vibration must include excitations induced by engine governor. ZF cannot accept any liability for gearbox noise or for damage to the gearbox, the flexible coupling or to other parts of the drive unit caused by torsional vibrations. Contact ZF for further information and assistance.
And from the ZF Repair Manual: ZF25A, ZF25, ZF45A, ZF45-1:
Troubleshooting
14. Chattering transmission noise mainly at low engine speed
14.1 The engine or propeller generates torsional vibrations in the drive unit which produces a hammering noise in the transmission.
14.1 Mount a flexible coupling with a different stiffness factor between en-gine and transmission; a coupling with a higher stiffness factor might be sufficient. Otherwise analyze the torsional vibrations to find out the re-quired stiffness for the coupling.
If your damper plate is OK (no loose springs), perhaps a flexible coupling might help mitigate some of the noise. Just a thought