Gen 2 3GSE Strange Noise

hey all,
Recently engine started making a strange noise.. its odd, had a listen but have no idea what it could be. basically the noise isn't there while its idling or accelerating but when the revs drop from say a gear change or just sitting there blapping the throttle a bit then revs dropping.. the sound is like a loud vibrating sound which is there for a few seconds then goes away and idles fine. later today ill try to get a recording of the sound.. but yeah if anyone might know what it could be or things to check... tbh it kind of sounds like near the timing cover or the extractors.. but thats just a guess.
 
cheers,