Since RAC 12.1 there is an additionally GIMR (Grid Infrastruktur Managament Repository) installed.
This DB is designed to be self-managed without DBA attention.
But it seems that sys-auditfiles were not be purged in regular manner. Over two years the audit files occupies 6GB in the grid base (/u01/app/grid/admin/_mgmtdb/adump).
The only way to housekeeping that is to remove old audit files with "find /u01/app/grid/admin/_mgmtdb/adump -name "*.aud" -type f -mtime +20 -exec rm {} \;".
This method is valid for ASM audit files, too. See here: "Manage Audit File Directory Growth with cron (Doc ID 1298957.1)".
It would be nice and this is my idea that purging of sys-audit files of GIMR and ASM instances are performed automatically and without DBA/OS Admin intervention.