bigtow

Hi can you help, I have a 2.5 manual 2004 Outback. The gearbox has a pronounced grow or brrrrrrrrrrrr as I accelerate slowly, not so bad if I accelerate faster. I had a 2ltr normally aspirated Forrester that didn't make this noise, is it unusual on the 2.5. A mechanic said not, but whist not loud it is iloud enough to be commented on by passengers. Should I ignore it or just speed up my driving to suit the vehicle. Thanks