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Actually I have noticed a new problem. When I try to start the car, it starts just fine, then I hear this sound (like a muffled thumping/clicking) from behind the firewall. Tough to describe the exact location, but it sounds to be higher up on the passenger side. I have no clue what it is, but the engine will then stall immedicately if I hear this sound. After about 5-6 times of trying to start it, the sound makes a definite click (like it resets) and the car starts and runs just fine. I do need a new driver's side cat (possibly oxygen sensors). Would this be the cause?

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Actually I have noticed a new preblem. When I try to start the car, it starts just fine, then I hear this sound (like a muffled thumping/clicking) from behind the firewall. Tough to describe the exact location, but it sounds to be higher up on the passenger side. I have no clue what it is, but the engine will then stall immedicately if I hear this sound. After about 5-6 times of trying to start it, the sound makes a definite click (like it resets) and the car starts and runs just fine. I do need a new driver's side cat (possibly oxygen sensors). Would this be the cause?

I don't think so. I do remember hearing about some valve at the back of the intake manifold which causes a clicking sound when it fails. Do a search on that. I can't remember who it happend to, but it was at least two of our members.