200mph

Yesterday, going over "the mountain" (Isle of man TT circuit) in my MY WRX 280ps(GC8D48D) I got the beginnings of a "I'm a rod and I wanna get out" sound. I'm am ex-bike mechanic and new to Subaru's since January.-I heard they were reliable!! (well at least I blew off the XK8!!)
Back to the problem...I nursed the car home but noticed no noise under load (accel or deccelloration) but in no load, just off the gas--clatter..
Is it what I think it is? Listenning to it with the bonnet up it almost sounds like a major exhaust blow.