High pitched rattle/vibration under acceleration

[CPUNeck]

When accelerating in first and second I can hear and feel a vibration under the car (drivers side) sounds like a cat shield, but can be "felt" through the floor board, shifter, steering wheel at times etc.

The sound oscillates with the engine RPM's meaning it comes and goes as the RPM's climb. But, it doesn't make any noise in the upper RPM's of the other gears (3rd, 4th, etc.) I should say I don't think the noise is there as I can't hear it and by then there is plenty of other noises being transmitted through the structure.

 

[Siren]

What jumps out at me, based on your description, is an exhaust leak. My 93 ATX had a similar issue, which turned out to be a leaky flex pipe in the exhaust system... if it's not this, then I'm really at a loss. When it first happened to me, it scared the heck out of me - my seat (and ass) were vibrating, and it carried through the rest of the structure of the car. Flex pipe replaced: issue resolved.

 

[itwonder]

May not be your problem but recent thread on vibration thats starts at about 4,000 RPM pointed to the clutch pedal spring.