When you twang a ruler close to the table there is less vibration so the sound is higher, when you twang it farther away there is more vibration so the sound is lower. Do double check just test for yourself.
The larger an instrument is, the more room inside it there is for air to vibrate. Because there is more space, it takes the vibrating air longer to vibrate through the entire instrument, which slows the overall vibration rate down. Slower vibration= lower pitch.
It is according to when you feel the vibration. If you have a vibration at higher speeds, your driveshaft could be out of balance. At lower speeds, you may actually have a cracked engine mount. Take it to a shop before it tears something up.