ISO9001/AS9100 says we "shall select auditors and conduct audits to ensure objectivity and the impartiality of the audit process".
Guidance doc ISO9002-2016 says "In some cases, specifically in smaller organizations or areas of the organization where specific job knowledge is required, it can be necessary for a person to audit their own work. In this situation, the organization might have the internal auditor work with a peer, or have the results reviewed by a peer or a manager, to ensure results are impartial. The organization could also consider obtaining resources from an external provider such as a university, external auditor, or another
organization."
The reality is, I know our registrar auditors well enough, having been through this kind of discussion with them before, that if someone audits their own work there will be hell to pay and it may well be impossible to convince them it is not a NC, short of maybe showing them where an auditor wrote up a NC on themselves. YMMV.