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So just recently made a trip 1800miles and around the last 300 miles I hear a rattling noise (I thought it was timing chain rattling) only when accelerating in 6th gear around 55-60 mph and 1500-2000 rpms, now when it goes past that I can’t hear it. Now the next day I take it out and it seems to do it slightly under load through some of the gears always low rpms . It’s not constant and does not seem to effect the driving but I know it’s not suppose to sound that way when accelerating .

Ls2 w/port n polished stainless steel heads, cammed, driveshaft, straightpipes, carbon fiber synchros, transmission and engine rebuilt 3 months ago and 109k miles on vehicle 7C53F071-ACF7-4C42-9C7F-0B9824DB05CE_1553226268368.jpg
 

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I know nothing of the newer cars/engines, BUT, if you described your problem as you have and it was an older Pontiac, I would say it sounded like detonation and would recommend having your timing checked - it may be too advanced OR the octane/gas you are using is too low if timing won't pull out the rattling (detonation). (Might also suggest a step or 2 colder spark plug due to the use of iron heads on the older engines)

Next thing could be a lean condition. Sometimes with carbs you need to fatten them up as the ethanol laced gas runs a little leaner by nature. Maybe you need a bigger injector size if you went with a bigger cam?

You may also want to have your engine codes checked even though you may not have any engine lights popping up. Could be a bad sensor somewhere that is not correcting the air/fuel ratio and or timing - and that is just a guess.

Continued detonation, if that is what it is, will damage/destroy an engine if left to go on too long. So that is what I would check first if it was an older carb'd engine. :thumbsup:
 

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Tremec transmissions have a tendency to chatter at low RPM under load. I have the T56 Magnum in my '06, and it has some low end chatter even with fewer than 1,000 miles on it. The OEM shift linkage, which has a significant back-swept design, and plenty of slop, isolated the of some of that. Seems you've had your transmission redone at some point (CF Synchros)... My first guess is this is the noise you are hearing.
 
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