New to subaru - 2005 Impreza Rs Wagon 2.5L

Hi everyone!
Just got hold of an 05 RS and loving it. It has got only 34k miles and missed 2 of its services. and also been garaged for 2-3years. hence low miles.
Upon acceleration in 1st if you get the revs around 3000 or higher before letting out the clutch it makes a really weird strong whining noise, mechanic said it is quite common? does anyone have any idea what could be making that noise and if it is serious?

Thanks a lot