merlot97

hi all, just bought a 97 sti import last week, i noticed that between 3000-4000 rpm the car makes a strange howl, like a vibration possibly from turbo?
the dealer i bought it from looked it over before i picked it up, and serviced it, but couldn't find what the noise was or from where it came. he said it wasn't pronouced enough to worry about, and to be honest it drives perfectly for a 9 yr old wrx sti. should i be concerned or is it just age?