I had this problem when a put my 2 rf p3s in a box my buddy gave me because the one i built was too big for the trunk but on lows i would hear the same sound ur hearing and on the higher notes it wouldnt rattle but i thought it was my amp and i would push on it and it would go away but it was back of the box had no screws in it so it would rattle i would check your box and port to see if theres secured