I Finally Found And Corrected My Rattle.

RVJ · March 29, 2008

Since I bought my truck with the 5.3L, it has had an annoying rattle that is only there when it's in 4 cylinder mode and under very light acceleration. It kind of sounds like a helicopter fluttering. I knew it was from underneath about midway towards the front. That is the only complaint I've ever had with this most excellent truck.

After washing it today I thought I would crawl under and have a look see. I found that there is a shield on the passenger side of the transmission that has 2 bolts holding it on to the side of the tranny. I think it's there to keep the heat off of the tranny because the exhaust pipe is right beside it. It is too close to the tranny pan and under the right harmonics it resonates against the tranny. I simply bent it away fron the tranny from front to back, only bent it enough so that it wouldn't touch the tranny, and now the noise is gone. I can't believe I'm alone in having this noise as they are all alike I'm assuming.

TexasAl · March 30, 2008

Interesting. I haven't had this noise, but I'm sure other have experienced it. If I ever hear such a noise, at least I'll know where to look first. Thanks.

-Al

hubbycap · March 30, 2008

You are not alone Bob. I feel your pain. I too have experienced the flutter you speak of but have been too shy to bring it up. I admire your courage in admitting this publicly. Just kiddin'.

Really, I've noticed the flutter and thought that it was the nature of the beast. I do feel this harmonic in my accelerator foot, not sure if it is the tranny hump or the pedal. Your discovery seems to confirm a correlation of that area. I'm enjoying a couple of cocktails this evening, but tomorrow I'm going to roll underneath Silvia and check that out. That flutter, though slight, is the only way I know that the V4 mode is operating outside of the DIC (almost the worst possible choice of an acronym.) Your sleuthing is greatly appreciated.

RVJ · March 30, 2008

It's been making me flutter since I drove it off the lot.

98 Z71 SLT · March 30, 2008

I, too, have been severely annoyed by this AFM flutter/vibration on my 2008 Sierra.

I will be crawling under the truck in the morning, and post info here.

thanks for the info !!

saxdogg · March 30, 2008

Hmmmmm.....mine won't "accelerate" the least bit in v4 mode...it kicks out at the slightest throttle touch or hill. All is seamless.

VMax2007 · March 30, 2008

Hmmmmm.....mine won't "accelerate" the least bit in v4 mode...it kicks out at the slightest throttle touch or hill. All is seamless.

Same here. If not for the DIC, I would never know I have AFM, it is 100% seamless and undetectable. And the ONLY time it's on is when I am decelerating or cruising constant speed on the freeway with no load, incline or headwind and I am under 65 mph. Heck, once when I was on a flat freeway cruising 60 mph, it was in 4 cyl mode, I turned on the A/C and it kicked into V8 mode.

svensk · March 30, 2008

Hmmmmm.....mine won't "accelerate" the least bit in v4 mode...it kicks out at the slightest throttle touch or hill. All is seamless.

Same here. If not for the DIC, I would never know I have AFM, it is 100% seamless and undetectable. And the ONLY time it's on is when I am decelerating or cruising constant speed on the freeway with no load, incline or headwind and I am under 65 mph. Heck, once when I was on a flat freeway cruising 60 mph, it was in 4 cyl mode, I turned on the A/C and it kicked into V8 mode.

I'll kick you into v8 mode.

hubbycap · March 30, 2008

Creeped under mine this AM. Bumped on it and didn't notice a rattle or anything. It was pretty close to the tranny case on the engine end and I tweaked it out slightly, maybe 1/8". Exceptionally stiff for a heat shield. I'll try to pay close attention for any change in sound today.

hubbycap · March 30, 2008

Ran some errands and worked out a nice interstate leg on the trip. Very difficult to tell when V4/V8 switches were made. Sometimes I thought it had and it hadn't! The heavy gauge of the tranny heat shield must have been responsible for the lower frequency "flutter," thinner sheet metal would generally resonate at a higher pitch. Harmonics might be the very reason for the heavier gauge. I'm sure GM wouldn't have spent an extra 57 cents if it had not been necessary.

RVJ · March 30, 2008

That's what I thought, very thin and flexible. Easily bent also. I squeezed and released and when it first sprang back into position it gave the flutter noise just sitting there. Made me a pretty happy guy after that.

norcalZ71 · July 12, 2008

Since I bought my truck with the 5.3L, it has had an annoying rattle that is only there when it's in 4 cylinder mode and under very light acceleration. It kind of sounds like a helicopter fluttering. I knew it was from underneath about midway towards the front. That is the only complaint I've ever had with this most excellent truck.
After washing it today I thought I would crawl under and have a look see. I found that there is a shield on the passenger side of the transmission that has 2 bolts holding it on to the side of the tranny. I think it's there to keep the heat off of the tranny because the exhaust pipe is right beside it. It is too close to the tranny pan and under the right harmonics it resonates against the tranny. I simply bent it away fron the tranny from front to back, only bent it enough so that it wouldn't touch the tranny, and now the noise is gone. I can't believe I'm alone in having this noise as they are all alike I'm assuming.

Hi RVJ: I'm having a drone or hum noise too and sometimes a slight vibration in the gas pedal/floorboard. Maybe your post is a solution for me! I have couple questions though....

When you bent the shield, did you bend it towards the exhaust pipe or towards the tranny pan?

Can you explain more in detail what exactly and how you bent it? thanks for your help!

slimjim2525 · July 12, 2008

No similar noise for me.

CalixEros · July 13, 2008

Since I bought my truck with the 5.3L, it has had an annoying rattle that is only there when it's in 4 cylinder mode and under very light acceleration. It kind of sounds like a helicopter fluttering. I knew it was from underneath about midway towards the front. That is the only complaint I've ever had with this most excellent truck.
After washing it today I thought I would crawl under and have a look see. I found that there is a shield on the passenger side of the transmission that has 2 bolts holding it on to the side of the tranny. I think it's there to keep the heat off of the tranny because the exhaust pipe is right beside it. It is too close to the tranny pan and under the right harmonics it resonates against the tranny. I simply bent it away fron the tranny from front to back, only bent it enough so that it wouldn't touch the tranny, and now the noise is gone. I can't believe I'm alone in having this noise as they are all alike I'm assuming.

Hi RVJ: I'm having a drone or hum noise too and sometimes a slight vibration in the gas pedal/floorboard. Maybe your post is a solution for me! I have couple questions though....

When you bent the shield, did you bend it towards the exhaust pipe or towards the tranny pan?

Can you explain more in detail what exactly and how you bent it? thanks for your help!

I get the same feeling in the gas pedal. I chalked it up to a quirk of AFM with the engine struggling while only running 4 cylinders right before switching to V8 mode. Although I've only noticed it under heavy load in V4 load, not light as the others have found with the heat shield.

RVJ · July 13, 2008

I also found the black motor cover was vibrating on a small wire loom. A couple holes and wire ties silenced in up nicely. I mainly noticed mune at about 45 mph in v4 mode and when accelerating slightly and going into v8 mode. It's all quite now.