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i agree on having someone drive it who is familiar with them.....
if you are in gear, and stand on the throttle does it clunk? that is the telltale sign.....it'll feel like someone is hitting the car underneath just behind the front seats with a rubber hammer.....
but as i've said, i've driven quite a few 997's and they just feel like that if i understand your description...
they aren't hard to change......maybe a 1/2 hour with a lift and air tools...