First of all, is that REALLY an STI? You have no STI fog light covers and you switched to a stock WRX spoiler? Why would you want your STI to look like a WRX? I have an '05 WRX with an STI spoiler but I'm not converting it to look like an STI ### it can't quite stack up to that competition. I'm trying to make my WRX BETTER than a stock STI. That's my goal.
As for the rumble...why don't ya just take it back to the installers, unless that was you, in which case take it back to the Subaru dealership and get some professional help.