Usefull topics from the forum and other sides I found so far:
A topic about the original fans and options. I ordered most of these fans (and some others) and will provide data as soon as I have tested various combinations and/or alternatives.
A topic about a bigger and better cooling fan. This is mainly about improving print quality, but I think it will provide some noise reduction because of the larger and most times quieter fan. Will be tested as soon as parts arrive.
An alternative backplate for a 40mm PSU case fan. This is a valid alternative, especially because the PSU case provides enough space for a 40mmx20mm fan which is a lot quieter while providing more airflow.
An alternative workaround for a bigger PSU case fan I found in a topic here in the forum (not sure which one, already lost track 
). I seams quite nice, I will also test this one, but might use a wider fan (40x20mm).
Other work-in-progress projects:
- Reducing the transmitted vibration with foam pads or rubber granulate pads.
- Soundproof the enclosure with acoustic foam.
- Bigger 12V PC fan for enclosure exhaust (more airflow, better regulation, less noise).
- Noise reduction effect of an airfilter for the enclosure exhaust (is also important for me because I can’t exhaust outside.
Please give me some time to test all this things, if you already use some of these or different approaches or have ideas for tests send me a pm or post in this topic!