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Discussion Starter #1
2005 3.5 SE automatic

45k miles

never heard this before (came out of no where)

40 miles/50 minutes later (seemed normal)


oil change maybe 3k miles ago but 2 years ago (synthetic)

oil level high out of no where, will check again tomorrow

no smoke
 

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The only thing that occurs to me is piston slap. The audio doesn't sound quite right for a slapper, but that might be the quality of the recording. Slap is generally the only mechanical noise that disappears when the engine warms, because it's caused by excessive clearance on a skirt that disappears as the piston warms and the skirt expands. On an OHC, pretty much all other mechanical noises inside the engine will get worse as the oil thins. If it does it consistently when cold, put a stethoscope on it. If it's slap, you'll hear it loudest on one side around midway up the engine block.
 

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this morning (Was 56f), started normal, just got oil changed anyways, we'll see if I can get it to repeat

1st time in 15 years heard that on startup, not sure what could have caused it

did drive in a rain/snowstorm a few nights before and not sure if maybe drove through too many puddles/snow

previous night heard a ton of crackling under the car probably from some thermal expansion, but no clue if related

went on the justanswer, they thing to have narrowed it down to:
timing chain/tensioner
cam sprockets
oil filter
lifters
and/or exhaust shield
 

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Too loud and rhythmic to be an exhaust shield, and a lifter with a bad seal should get louder with thin oil, not quieter. Tensioners and guides on the 3.5's do wear out, but they make a characteristic "bumblebee" noise that I'm not hearing, and they also won't be temperature-dependent. Maybe you had something stuck behind one of the pulleys? I dunno...
 

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Discussion Starter #5
Not sure, sound has since gone, granted its warmer here (50s), will wait for a colder day to see if it re-occurs.

Today ran quiet, hoping it doesn't come back, but mystery is on the back of my mind.

Appreciate your help by the way
 

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You're most welcome (and Happy New Year)! It just occurred to me, did you inspect your drive belt? Most of the 3.5's have a metal guard to protect the Bank1 IVT against "whipping" from a shredding serpentine belt, and that noise sounded suspiciously like metal being slapped. See if your belt has any missing strips or chunks.
 

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this morning (Was 56f), started normal, just got oil changed anyways, we'll see if I can get it to repeat

1st time in 15 years heard that on startup, not sure what could have caused it

did drive in a rain/snowstorm a few nights before and not sure if maybe drove through too many puddles/snow

previous night heard a ton of crackling under the car probably from some thermal expansion, but no clue if related

went on the justanswer, they thing to have narrowed it down to:
timing chain/tensioner
cam sprockets
oil filter
lifters
and/or exhaust shield
When you had that noise, was the oil light on the dash turned on? If it was on for that period of time, a loss of oil pressure could have caused the chain tensioners to back off making the chains loose and rattling. If no oil light, then something on the outside of the engine probably caused the noise. Driving in a rain/snowstorm, it's very possible you picked up some debris which might have got stuck in the alternator and then finally fell out. I've encountered similar noise due to bad bearings in the alternator.
 
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