Officers are looking into what happened during the early hours of Monday 12 August in the areas around Union Street and Allez Street, off the Grange.
Guernsey Police say there was an assault, but no details have been given about who was responsible, the victim or any injuries caused.
A motor vehicle was also damaged during the same incident, which started in Union Street before continuing in Allez Street. It all started at around 01:00.
In a statement, Guernsey Police said:
“The incident involved loud noise and would have got the attention of anyone in the area at that time. We would appeal for anyone who may have seen or heard the incident to come forward.”
Any witnesses should contact PC 17 Kusimo in the Investigation Support Unit on 725111 or through Crimestoppers anonymously on 0800 555 111.