What kind of flutter are you talking about. If you're talking about a flutter in the exaust note than yeh it's absolutely normal. If it's a metalic noise than you could be vibrating against anything. Check your bolts to the s pipe like he said and try to re position the MIDPIPE to avoid the rattling. I hope this helps.
I think its normal... happens to my cousins EVO and my friends WRX.... its just the sound of the car shutting off i think... and you dont have a cat no more like u said to deaden the sound.