little vibration in steering, pad and accelarator

I have LT trailblazer 2006. My friend was checking this car. He put his feet on the brake and accelarator in the same time thrice. He told me he is checking the mounts. After that time their is a slight vibration in the car seats, steering, pad and accelarator at low speed. It disappears some time also, what could be the probable cause?

No way to accurately diagnose this over the net, look for any under chassis interference, exhaust pipes and shields in particular. Why check mounts to begin with?

There was no vibration even before he checked it. He was checking it because he wanted to buy that car. After this this vibration has started. Also this happens at very low speed i.e 15-30 MPH. What disturbance can be caused when you push accelarator and brake in the same time which can produece vibration afterwards?

The engine torques or moves slightly in the chassis, if there was too little clearance of anything attached to the drive train it could contact the frame or body.