2015 Yukon XL Denali Low acceleration grumble sound

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Russ Giles

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Installed a Magnaflow Cat Back exhaust with a K&N filter. On the way home after rounding a residential corner and applying minimal throttle I heard aloud grumble (hard to describe) sound under the vehicle like a blockage was happening or the flow was restricted just for a few seconds then gone, very annoying. Doesn't happen under full acceleration or when parked trying to recreate.
happens between only 1,500-2,000 rpms with light throttle only
More regularly now
Clammed the flapper valve full open - no change
Turned off v4 mode and did a Diablo 87 octane tune - no change
Took it to my muffler guy, who replicated the sound on a test drive but says it is not the exhaust. plugged any obvious, small leaks
Nothing loose, nothing touching the heat shields, no obvious loose pieces under the vehicle
Can't figure this one out. Motor mounts? Harmonics? what else can it be?
 

STORMIN08

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More than.likely it is just throttle input sounds...I trim my factory airbox and the tone is very noticeable...
 

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Russ Giles

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No this is a metallic grumble/vibration sound you can hear/feel below the driver and passenger seats. I have a feeling it is the trany going out.
 

STORMIN08

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Oddly enough...the flapper in the exhaust can come loose and flop...also causing a resonance that travels through the exhaust...

Maybe check that first, my 17 was weaker than I cared for at 60k is miles.and got a hose clamp around it, tightened in the open position
 
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Russ Giles

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Took it to a recommended Transmission shop. They put a scanner on the OBI port and we took a test drive. Turns out the impeller and clutch were slipping hitting over 200rpms when engaged which was causing the noise. T Converter shot and a full rebuild is needed, $4.5k is the estimate. New kevlar clutch plate and stainless other components cool but a bummer to have to spend so much. Trany guys says that EPA mandated that GM run trany hotter to reduce emissions which fries the hydraulic fluid so he is going to install a coupler to reduce the temp down to 140-150 degrees. will post once it is complete.
 

STORMIN08

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look into the trans line mod, it allows fluid flow all the time, most report temps only reaching 150-170 in normal conditions
 

OmegaRoach

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Took it to a recommended Transmission shop. They put a scanner on the OBI port and we took a test drive. Turns out the impeller and clutch were slipping hitting over 200rpms when engaged which was causing the noise. T Converter shot and a full rebuild is needed, $4.5k is the estimate. New kevlar clutch plate and stainless other components cool but a bummer to have to spend so much. Trany guys says that EPA mandated that GM run trany hotter to reduce emissions which fries the hydraulic fluid so he is going to install a coupler to reduce the temp down to 140-150 degrees. will post once it is complete.
I'm getting the same noise, smh. Currently at ~116k miles.

Thanks for posting this.
 

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Took it to a recommended Transmission shop. They put a scanner on the OBI port and we took a test drive. Turns out the impeller and clutch were slipping hitting over 200rpms when engaged which was causing the noise. T Converter shot and a full rebuild is needed, $4.5k is the estimate. New kevlar clutch plate and stainless other components cool but a bummer to have to spend so much. Trany guys says that EPA mandated that GM run trany hotter to reduce emissions which fries the hydraulic fluid so he is going to install a coupler to reduce the temp down to 140-150 degrees. will post once it is complete.
a coupler? haven't heard of this, do they have a product name or part number? these do have a 180deg external thermostat which is easily bypassed which will lower the temps about 20 deg but depending on how you drive that alone won't do much, you might want to look at a trucool 40k. 4.5k is a lot unless they are doing some serious upgrades, just had one of mine done for just under that built for 500HP
 
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