When the trailblazer is in drive , foot on brake and you throttle to 900 RPM there is a very harsh vibration noise coming from somewhere in the center of the vehicle up front. Could this be motor mounts or a transmission mount? I had just replaced the whole front end on the vehicle today. I don't suspect the control arms or tie rods. It does not do it in reverse with the foot on the brake. When at a stop sign and you hit the accelerator that vibration is present just for a brief moment as it passes that 900 RPM.