Random display of listings

Hi there
I would like my listings to all be displayed randomly by default. I have set the default to “System Set (no default order)” but all listings are displayed newest to oldest unless I change it in the dropdown.
I tried adding a custom orderby option using the code Random [orderby=rand] and then was hoping to set it to default but everytime I select this orderby option my search changes to 0 results.
Please could someone point me in the right direction.
Thanks so much
Veronica