smi2ml

Hi, I have a subaru sti 2007 and at 5500rpm it loses power and makes a loud rickety noise, I took it to a garage and they said its detonating! I have got it booked in at marlin motors on Tuesday and hopefully they will be able to fix it for me as they have a 4wheel drive rolling road, I use tesco 99 Ron and shell vpower. Has anyone else had this problem? Does anyone know what may of caused it? Thanks in advanced for your help