It's your exhaust. My Motordyne Shockwave exhaust makes fluttering pops on deceleration/downshifting. Since I replaced the cats with Kinnetix High Flows, they occur more frequently and sound deeper.
I have Toyo Proxies 4 Plus all around on my '09 370Z
I'm running Toyo Proxes 4 Plus all-seasons on my '09 370:
245/35/20 on 20x8.5 TSW Tabacs front
295/30/20 on 20x10 TSW Tabacs rear
I've had the Proxes for awhile now. They're fairly quiet, grip well year round and have excellent tread life.