not to sure to make of this one

my xsi has been sat around for a yearish now (started every other month). dusted off the cobwebs

and all is well apart from some rust........

and this noise.....
when i let off the throttle after reving....

as the revs spool down its like it makes a weird rattle. kind of like a flappy exhaust shield rattle mixed with a lawnmower bell clutch type ring. its not the exhaust heat shield, although it louder and more pronounced with my head at the rear of the engine

car starts runs a stops fine..... so im a little stumped

thanks in advance for the help
jim

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