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I saw a thread regarding this months ago, either on this forum or another. Many blamed the IAC valve, vacuum leaks etc. Some seemed to resolve themselves to accepting that their car was just noisier than others.

I discovered something that these people should check out. Yesterday I held my hand near the exhaust header and could feel a slight breeze. I shut the engine off, removed the hanger bracket, and tightened each exhaust header bolt by about 1/16 of a turn. I no longer feel that "breeze" near the exhaust, and the engine is noticeably quieter now.

My rationale that the engine would start noisy, then become quieter as it warmed up is that as the engine warms, the headers expand slightly to close any gaps to the block thereby sealing exhaust and noise.

I'm thinking it could be time for new exhaust manifold gaskets, but I can tell you I'm not looking forwards to doing the exhaust header gaskets for cylinders 2,4, and 6.

The only reason I'm posting this is for those who experience excessive engine noise, do look for typical vacuum leaks etc, but look for exhaust leaks at the headers, and exhaust crossover pipe too. Getting the exhaust crossover pipe to seal against the donut gasket is rather tricky and I had a bad leak there after doing my LIM/UIM job.
 

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Maybe before you buy any parts, try loosening up and retightening all the manifold bolts without removal. This may stop the leak without spending $. If you do this, use some WD40 or similar rust penetrant on the manifold bolts before you start loosening. Jake
 
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