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Exhaust making weird sounds

Exhaust making weird sounds

Postby jamestaylor47 [OP] » Feb 19 2013, 4:59pm

On my 07 Sierra 5.3 gas I notice when I get to speeds above 40 and set the cruise everytime it starts to drop rpm going down a hill ect a weird sound comes out the exhaust Similar to a sound of when a semi downshifts then once it picks the Rpms back up it quits. I took the pipe off and it still did it. Any ideas?
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Re: Exhaust making weird sounds

Postby 09chevyman » Feb 19 2013, 5:04pm

Is it a nbs(99-mid07) or nnbs(mid-07 and up). If nnbs it may be AFM kicking in (running on four cylinders).
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Exhaust making weird sounds

Postby jamestaylor47 [OP] » Feb 19 2013, 8:48pm

It's nnbs. And also forgot to me mention I stick my hand in the exhaust where I took it off and black oil was around it
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Re: Exhaust making weird sounds

Postby Pwgsx » Feb 28 2013, 11:03pm

The oil I dont know but it is going into v4 and it makes a horrible drone sound. Mine does it and I have it dumped and I think I will running pipes out the back soon, I hate the noise.
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Exhaust making weird sounds

Postby jamestaylor47 [OP] » Mar 01 2013, 9:14pm

As do I lol. So you can't put aftermarket pipes on a nnbs?
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Re: Exhaust making weird sounds

Postby dbrandt2009 » Mar 01 2013, 9:56pm

I agree it's probably the terrible AFM. I have aftermarket exhaust and when it kicks down to V4 it sounds like a dubstep song and "wobbles" a lot in sound. It gives me a headache
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Re: Exhaust making weird sounds

Postby DIESEL-24 » Mar 01 2013, 10:10pm

I have an idea of what it may be.. you say its a NNBS correct? And I noticed you say it sounds like a semi downshifting when your going down hill correct?? It could be your "Grade Braking". the next time it does it, look at your information cluster where it usually would display your odometer or your fuel economy.. if it is your grade braking it will say "grade braking" in the area where it normally displays your Instant ECON and Odometer and such.. mine does it every day when im coming home from work.. I have a section of road that is a long downhill plane.. i take my foot off the gas and coast down it and after severl seconds the truck will automatically down shift help you brake and stop the truck from gaining unwanted speeds.. as soon as you touch the gas it stops doing it. Its a feature on your truck. I could be totally off but thats what it sounds like to me from what you described. next time it does it look and see if its grade braking.. let us know.

P.S. I hear a lot about people hearing and feeling when the AFM kicks on and off.. when mine kicks on and off I dont hear anything and I dont feel anything.. not at all
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Re: Exhaust making weird sounds

Postby Pwgsx » Mar 01 2013, 10:47pm

mheuker wrote:I have an idea of what it may be.. you say its a NNBS correct? And I noticed you say it sounds like a semi downshifting when your going down hill correct?? It could be your "Grade Braking". the next time it does it, look at your information cluster where it usually would display your odometer or your fuel economy.. if it is your grade braking it will say "grade braking" in the area where it normally displays your Instant ECON and Odometer and such.. mine does it every day when im coming home from work.. I have a section of road that is a long downhill plane.. i take my foot off the gas and coast down it and after severl seconds the truck will automatically down shift help you brake and stop the truck from gaining unwanted speeds.. as soon as you touch the gas it stops doing it. Its a feature on your truck. I could be totally off but thats what it sounds like to me from what you described. next time it does it look and see if its grade braking.. let us know.

P.S. I hear a lot about people hearing and feeling when the AFM kicks on and off.. when mine kicks on and off I dont hear anything and I dont feel anything.. not at all


Do you have a aftermarket exhaust thats dumped?
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Re: Exhaust making weird sounds

Postby DIESEL-24 » Mar 01 2013, 10:53pm

No Pwgsx... i dont.... I can see where that would make a big difference.. I am putting on Flowmaster super 10's soon.. Im kinda scared on what thats gonna sound like now..
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