Just my .02 but I think its normal if your running a puck clutch. I have a similar issue with my ka-t setup. I had spec before and had crazy vibration around 3.5-4k on decel, did literally everything from mounts, bushings, driveshaft and even cracked open the bell housing and everything checked out.
Just rebuilt the motor and had the whole rotating assembly balance with comp bunny clutch for more power and right after starting up the car and driving down the block the noise was still there but less aggressive.
Its just "racecar" problems and something you have to deal with, running a puck clutch in any sorta car will make crazy noise having less dampening and stuff outside of oem shifter, mounts, flywheel etc will just make it louder than it should be.
Does it sound like this? Asked the guy and he told me the exact thing just being the puck clutch and is "normal" @17:33