The frame flex i'm thinking about is indeed real, i.e. putting strut bars and such on cars to make the frame more rigid, but as for a newer car that probably isn't taken to a road course all the time, it seems wierd. I've never heard of injecting anything in the frame though since the frame really isn't even that thick enough to inject anything into. most of the unibody frame is just sheet metal with the exection of the 2 frame rails that run the length of the car and the subframe itself. In all honesty, I would check your motor mount and your subframe bolts becuase if it only makes the noise when you get on the gas, the problem is coming from where the motor mounts to, i.e. MM and subframe.
