incomplete sysaux expdp unload (ORACLE)
Actually SYS.AUD$ table CAN be exported but only using old export command exp instead of expdp.

exp \"/ as sysdba\" file=aud_table.dmp tables=sys.aud$;


Since 11g or 10 (not sure) AUD$ table is prohibited to be exported.

The most convenient way is what @Balazs Papp suggested, to create table AUD$_BACKUP as select * from SYS.AUD$