2005 Cadillac SRX Noise and Car won't start

My engine light came on and the car started to shake. I stopped and had a diagnostic scan done at auto zone and they they it indicated that the crank shaft sensor was bad. The next day I was going to take it to the shop to have it repaired and as I turned the ignition, it started to make a loud sound as if a chain or something was in the engine. It would turn but not start and the chainning sound was very loud. Can anyone advise me what I should do as the dealer is telling me that it could be the engine or timing chain but they haven't even looked at it yet. They are also telling me that the repairs could run me between $5,000 to $6,000 dollars if it is the engine and as much as $1800 if it is the timing chain.