Quick update from my side, as I did quite a few tests trying to narrow this down.
I originally suspected the dual extruder/retraction mechanism, so I replaced the entire Dual Extruder Assembly with a spare one that Bambu had previously sent me for a completely unrelated issue. I then printed exactly the same model with the same settings. The clicking was identical, so replacing the whole extruder made no difference.
I also tested repeated manual extrusion and retraction with the toolhead stationary. In that case it was completely silent.
Then I printed the same model again with Retraction Length = 0 mm. The clicking disappeared completely.
To separate retraction itself from Z-hop, I restored the normal retraction settings and disabled only Z hop when retract by setting it to 0 mm. Again, the printer was completely silent.
So in my case the clicking is clearly related to the Z-hop movement, not the filament retraction itself.
I then changed the Z-hop type to “Normal” instead of the default behaviour. This helped a lot. The clicking is still present, but it is much quieter and no longer sounds nearly as aggressive as before.
With the quieter “Normal” Z-hop, I can also localize the remaining click better, and it seems to be coming from underneath the printer.
One more thing I noticed recently is a slight rubbing/scraping sound from the Z-axis/belt area when the bed moves up or down. I already performed the Z-belt tensioning procedure, but it did not change anything, so I’m not sure yet whether this is related.
For now, using Z-hop type = Normal is the best workaround I have found.
If anyone else with the same clicking noise can test the same settings, it would be interesting to compare the results