Thats the only way l can describe it......its a deep rumbling sound.......
Thanks, l thought thats what it was, but all subarus dont have boxer motors do they?

Because of the design of the motor. Its called a boxer and it is a horizontally opposed motor. Same design as porsche. All boxer motors have a unique noise.

yes they do, what year and model are you referring to?

The rumble does have some to do with the boxer motor but it is mostly because of the header design. In the US we get unequal length headers which cause the exhaust to exit in pulses. That is why you get the rumble. In other markets overseas, the ST l is come with a twin scroll turbo which requires equal length headers and they do not get the same rumble we get here.