Holden Barina 2003: Groaning gearbox

My 2003 Barina had the gearbox worked on under warranty in the middle of 2008. Last week it developed a groan from the gearbox under overdrive and when changing down. I checked the gearbox oil level and found it was down and it took nearly a litre to restore it to the full mark. It is now quieter, but still makes some noise, especially in reverse. The one Holden dealer has done all of the service work. Should I be concerned?