After installing my mac mid length headers I now feel this vibration at my gas pedal(well from under the car) like if something is hitting, I feel it more when I have a passenger in the car and when I accelerate hard.Could it be that the y-pipe they gave me is hitting something under the car?

 

Yes. If you look under the car you should see the y-pipe touching the frame. I'm going to try to wedge some radiator hose in between the frame and the y-pipe to see if that will fix it.

 

I had the same problem. We ended up cutting off the part of the Y pipe flange that was hitting the car. Putting hose there will probably take off a little of the vibration, but it will still be there till you gain some clearance.