Update #2 and FINAL
The problem seems to be solved once for all, I hope.
So all the suspension are ok and there isn't any oil leakage, the mechanics checked according to a Lexus instruction bulletin, and in the end they found the source of the goddamn noise.
They fixed the parking brake shoes (not the cable), as it was indicated in a second bulletin, and from that moment the noise was totally gone.
We tested the car on every single pothole, bumps and rough asphalt at various speeds and with different spring stiffness (NORMAL --> SPORT S --> SPORT S+).
The work has been done in like 1 hour (inspection included), without an appointment, but since they haven't released me any document I can't post anything "technical" about the problem nor a scan.
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Basically, the known reasons (at the moment) are that this noise may be caused by:
1) Oil leakage from a suspension
2) Parking brake shoes
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Hope this will be helpful.