It is habitual that periodically we find post about problems related to sys.aud$ table. Most of then are related to excessive growth of this table, you can truncate/shrink aud$ table, but you can't shrink SYSTEM tablespace...
To avoid space problems in SYSTEM tablespace, could be a good idea, that AUD$ table will resides out of SYSTEM tablespace ( in SYSAUX or in her own tablespace...). Today moving the table out of the SYSTEM tablespace is not supported.
My idea is to ask that aud$ table doesn't reside in system tablespace.
Regards-