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Discussion Starter · #1 ·
I have a 2003 400ex that makes a rattling noise. Valves are in spec, brand new tensioner, timing chain and guides. Brand new valves and springs. Brand new cylinder and piston. Any ideas what the noise could be?
 

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I have a 2003 400ex that makes a rattling noise. Valves are in spec, brand new tensioner, timing chain and guides. Brand new valves and springs. Brand new cylinder and piston. Any ideas what the noise could be?
Depends on the degree of rattle and what part of the motor it is coming from - could be the auto decompressor if in the top end - could be the clutch, transmission, starter drive, qalternator, crankshaft or counter balance if in the lower end
 

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Discussion Starter · #3 ·
Definitely not coming from the bottom end but I can’t seem to pinpoint where the noise is in the top end. It’s definitely pretty loud if it wasn’t noticeable I wouldn’t worry about it. When I give the quad gas the rattling gets louder. I have heard a lot about the auto decompressor. Not sure how to narrow it down.
 

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Out of curiosity how could the alternator cause a rattling noise?
The starter clutch is bolted to the alternator rotor - I have seen the bolts come loose and the clutch rattles on the rotor.

If the rattle gets louder as you open the throttle giving the engine more air - is it possible yhou are hearing spark knock caused by old or low octane fuel? You should be running 91 to 93 octane non-ethanol fuel.

I can't comment on the 'new' parts as I do not know the brand, bore size, piston to cylinder clearance, compression ratio or the technique used by the mechanic who did the work, but it may have something to do with the installation methods used.
 

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Discussion Starter · #6 ·
Well I am using 89 octane but I’ve always ran that without issue however maybe I will look into that. I used a niche piston and cylinder head 397cc when I rebuilt it.
 
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