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So I have never thought that I would be suspecting such widely different causes of a symptom on a car. Did the oil on the car on Sunday as well as the pads all the way around, nothing too crazy or intricate. Test drove it after the job was done and I hear a rattle coming from mid way down the car. Now I know noises from outside a car being bounced off walls into the car can be deceiving so I decided back in the air it goes and recheck the brakes. Everything checks out. While it was up I decided to run it in gear and man oh man its noisy as hell. Its a rattling that doesn't really increase or decrease with the engine RPMs or the wheel speed that I can tell.

However, the part I missed until the next morning is that the rattle doesn't manifest itself until the DOD kicks in. In manual mode I never hear it, but when the DOD engages it will rattle, and the second you give it throttle for the cylinders to begin firing on all 8 it goes away. The car is currently stock except a Magnaflow Catback. Looking under it, the flanges at the catalytic converters look like someone beat them with a hammer and shoved bubble gum into where the gaskets are supposed to be but I can tap, hit, beat, kick, and shake them to death and there's no noise.

So I have looked and searched and all that stuff and really I am not sure, I have new exhaust gaskets on order coming in tomorrow, I am debating changing the oil again because I decided to give the Rotella a shot (hopefully this doesn't spark a debate on running T6 in our cars. I researched and read up and opted to give it a shot.) And if its the worst, I am going to start ordering stuff for a cam and DOD delete.

So what could it be?
1 - Shotty exhaust job since before I got it, and when the DOD kicks in its resonating perfectly to make me think my motor is gonna explode from the rattle battle happening under my butt?
2 - I'm an idiot and putting Rotella T6 5w40 in my car is actually a bad idea and I should go back to Valvoline where I'm all comfy n stuff.
*Side note, the car has 8 quarts in it right now, I know the capacity is 8.8 and when I checked the stick it was dead in the center of the Min-Max dots on the stick. Ive worked on cars for years and usually dead center is never a cause for an issue but with the AFM and whatever other voodoo GM has in here is the .8qt making the insides of my motor rattle like bad skeleton Halloween decorations in the wind?
3 - Its time to join the rumble club and participate in every Thumpin' Thursday in the facebook groups til I send a rod through the block doing dumb stuff later on in the car's life because a DOD component has decided to warn me before committing Seppuku inside of my engine leaving shards of its empty soul all over the inside of it?

Any insight would be great, I have a video I can post up but the audio isn't amazing.
 

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Check the metal heat shields around your exhaust's midpipes. If it's loose, it'll rattle and hit up against the driveshaft. Had this very thing happen to me. Ended up just ripping it out completely. Haven't heard it since.
 

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Discussion Starter #3
Check the metal heat shields around your exhaust's midpipes. If it's loose, it'll rattle and hit up against the driveshaft. Had this very thing happen to me. Ended up just ripping it out completely. Haven't heard it since.
So have you had issues with heat considering the heat shield isn’t there? When I checked it it was loose but after tightening the bolts up it seemed secure-ish.
 

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So have you had issues with heat considering the heat shield isn’t there? When I checked it it was loose but after tightening the bolts up it seemed secure-ish.
Nope. That said if the bolt holes on shield weren't so damaged from the abuse it took after coming loose, I would have bolted it back on.
 

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So I have never thought that I would be suspecting such widely different causes of a symptom on a car. Did the oil on the car on Sunday as well as the pads all the way around, nothing too crazy or intricate. Test drove it after the job was done and I hear a rattle coming from mid way down the car. Now I know noises from outside a car being bounced off walls into the car can be deceiving so I decided back in the air it goes and recheck the brakes. Everything checks out. While it was up I decided to run it in gear and man oh man its noisy as hell. Its a rattling that doesn't really increase or decrease with the engine RPMs or the wheel speed that I can tell.

However, the part I missed until the next morning is that the rattle doesn't manifest itself until the DOD kicks in. In manual mode I never hear it, but when the DOD engages it will rattle, and the second you give it throttle for the cylinders to begin firing on all 8 it goes away. The car is currently stock except a Magnaflow Catback. Looking under it, the flanges at the catalytic converters look like someone beat them with a hammer and shoved bubble gum into where the gaskets are supposed to be but I can tap, hit, beat, kick, and shake them to death and there's no noise.

So I have looked and searched and all that stuff and really I am not sure, I have new exhaust gaskets on order coming in tomorrow, I am debating changing the oil again because I decided to give the Rotella a shot (hopefully this doesn't spark a debate on running T6 in our cars. I researched and read up and opted to give it a shot.) And if its the worst, I am going to start ordering stuff for a cam and DOD delete.

So what could it be?
1 - Shotty exhaust job since before I got it, and when the DOD kicks in its resonating perfectly to make me think my motor is gonna explode from the rattle battle happening under my butt?
2 - I'm an idiot and putting Rotella T6 5w40 in my car is actually a bad idea and I should go back to Valvoline where I'm all comfy n stuff.
*Side note, the car has 8 quarts in it right now, I know the capacity is 8.8 and when I checked the stick it was dead in the center of the Min-Max dots on the stick. Ive worked on cars for years and usually dead center is never a cause for an issue but with the AFM and whatever other voodoo GM has in here is the .8qt making the insides of my motor rattle like bad skeleton Halloween decorations in the wind?
3 - Its time to join the rumble club and participate in every Thumpin' Thursday in the facebook groups til I send a rod through the block doing dumb stuff later on in the car's life because a DOD component has decided to warn me before committing Seppuku inside of my engine leaving shards of its empty soul all over the inside of it?

Any insight would be great, I have a video I can post up but the audio isn't amazing.
I had a similar experience with what I thought was a collapsed cat, but on further investigation of the exhaust system we noticed that the an inspection cover on the trans was missing which is directly underneath the flexplate,so we had a look in there and low and behold we spotted what looked like a hairline fracture. Anyway we pulled the trans out and sure enough it had two of the three webs cracked,so I’d have a look at your flexplate, just my experience.
 
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