Slight metallic flutter when engaging

As the title says I'm starting to get a slight metallic flutter, quite high pitched and light that occurs when I pull off.



Doesnt happen when revving the car in neutral or with clutch depressed.



Its very difficult to notice, in fact I can only near it when I'm next to a building/wall that'll reverb and amplify the noises.



Bit vague but I'm hoping someone else has had this before? Any ideas appreicated.

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