The solution is in the last line.
I have connected a new Atari XF551 disk drive from USA to a PAL Atari 130XE.
The drive boots slowly (the sound is "beep - brrrrr - beep - brrrrr" instead of "beep beep beep beep") and sometimes with boot errors.
After having changed different PAL computers, SIO cables, floppy disks, PSUs, the problem remains.
Thanks to MEtalGuy66, I have discovered that the two SIO connectors at the back of the XF551 drives are soldered in place by the actual conductive leads and nothing else.
As a result of plugging and un-plugging cables over time, and since the conductive leads are only soldered to one side of the board, the solder joints crack and become unreliable.
My drive was supposed to be new but anyway Iíve sent the mainboard to MEtalGuy66 who has checked it and removed Atari poor solder and soldered the SIO connectors.
A perfect work, both aesthetically and functionally, but my problem still remains.
Then I have installed an Hyper XF eprom (also burned by MEtalGuy66) and the drive works (apart the fact that itís unable to format in DSDD with AtariDOS XE); this rom image works perfectly both in Europe and America.
A supposed to be PAL eprom didnít work (after 10 seconds the drive mechanic starts doing some horrible noises); Iíve found that it was an NTSC rom patched for a 3.5 drive!
Finally Iíve received from JR> a real PAL rom image (I attach it), classics has quickly burned it for me and now my XF551 works perfectly!
So, if you want to use an American XF551 drive with a PAL Atari, you have to change the original NTSC rom with an eprom containing the attached PAL image (or, if you prefer, an Hyper XF image).