At least once during each period of 12 months, the trust institution shall cause an audit to be made of the common trust fund by an auditor or auditors responsible only to the board of directors of the trust institution. The report of such audit shall include a list of the investments comprising the common trust fund at the time of the audit, which shall show the valuation placed on each item as of the date of the audit, a statement of purchases, sales and any other investment changes and of all income and disbursements during the period to which the audit relates. The period of the first audit shall commence with the establishment of the common trust fund, and each succeeding audit shall commence at the end of the period covering the preceding audit. The reasonable expense of any such audit may be charged to the common trust fund, and a copy of such audit shall be available at the principal office of the trust institution for inspection during all regular business hours to any person having an interest in a participation in the common trust fund; provided, that the trust institution shall be required to furnish a copy of such audit to any person having a share in the investments of the common trust fund without expense to such person should the interested person request a copy of such audit.