Rute-cause found and eliminated.
I've made some tests and I guess I found the "FAIL" and fixed it...
The problem comes when at start up the phone finds a message (sms/hotmail/email....) for which it shoud play a notification ringtone.
A couple of weeks ago I changed the ringtone to a "custom" I've placed in \MEDIA\audio\notification\ folder
Normally it works fine but at start up, the phone checks for messages WHILE still "scanning the media"....
... so if it needs to ring the notification before having scanned the \MEDIA\... folder it plays that annoying non-stop ringtone (probably a call ringtone).
I've changed back the message notification to one of the default notifications and the problem disappeared at once.
For Info: my \MEDIA\ folder is in the internal SD card.
The problem is no more there but I can't use my customised ringtone... any solution or suggestion?