Have this deacceleration noise also, v7 sti all stock standard so don't think flywheel theory is a given.
Only started few weeks ago after not using car for 2 months.
Had a quick look on web myself and no sure fix. Some mention bearing in transfer case, mounts, heat shields, pilot bearing, tire pressures! Etc etc.
Will get a shop to look at it next service.
Please post if you find a cure for yours. Will do the same. Good luck!