The official IRNA news agency reports no casualties and no damage.
The governor of the restive city, Mohammad Jaffar Sarami, is quoted as saying that the "sound bomb" went off shortly before midnight in the same area as one of the blasts last week.
Eight people were killed and 46 were wounded in the blasts on 24 January in Ahvaz.
Ahvaz is the capital of Khuzestan Province, with a large ethnic minority Iranian Arabs.
Iran's Arab Minority
The aftermath of a car bombing in Ahvaz on June 12, 2005 (epa)
DISCONTENT IN THE SOUTH: There are approximately 2.07 million ethnic Arabs in Iran (3 percent of the total population of 69 million), most of them living in the region around the southern city of Ahvaz. The region has been the scene of numerous terrorist attacks and demonstrations over the years. Arab groups cite historical grievances, and they bemoan inadequate attention to their culture and language by state media....(more)