Well it sounds like it anyway... - I just figured I would throw this out hoping someone knows what the heck it is I'm talking about. I was half tempted ...

I just figured I would throw this out hoping someone knows what the heck it is I'm talking about.

I was half tempted to put this in the trans/drivetrain area as I think that's where it's coming from but can't be totally sure.

Here's the deal. I have the 6 speed trans on my 2011 and when going slow and maintaining a 25-35 mph speed, the engine RPM is really low (1k ish) and when I gas it ever so slightly (not enough to downshift) I get this rattling / tinking sound. Best description is it sounds like a small amount of change rattling around in a glass jar. It appears to ONLY do this in the lowest (6th) gear which the RPM and speed seem to low for honestly. When I gas it enough to force it to change gears, the sound goes away.

This developed a few weeks ago (I think) and didn't do this originally. It is/was a new truck.

I’m open to the possibility it just happens to be some resonance point being hit causing *some* sort of rattle, but first impression is changing gears it goes away, and it doesn’t take major acceleration, just enough to get it to change gears and poof, it’s gone.

I know I maybe fishing in an empty pond but figured I’d give it a shot to see if someone’s seen this before.

what gas are you running mine did that sounds like it was between the motor and the tranny and they told me it was spark knock not sure but maybe try a higher test gas and see if it still happens

Had that thought once too but I've run straight 87, straight 93 and even a tank of E85 (105 octane) and it didn't make a bit of difference with any of the above.

Do you have stock size tires on it? It could be the engine is under too much load for the rpm, but it isn't reading the need to downshift based on speed. That may be causing some knock? It also may be a total normal thing under a very specific set of circumstances, which you seem to have found.

Sent from my Evo

Do you think it may be some loose gravel in a heat shield?

Shift the selector into M5 mode and drive it around and see if it still makes the rattle. At first read it sounds like engine ping but didn't want to speculate. You can drive all day long at 60 mph in M5. It'll shift up and down through the gears just like normal. It just won't go into 6th until you hit the + button on the shift lever.Whether the noise persists or not I'd bring it in under warranty as it doesn't sound normal. My 2011 5.3 engine Z71 4X4 doesn't do it.

I seem to have the same noise. It is difficult to describe. You said it precisely. I notice it at slower speeds under a slight acceleration. It is faint but a weird sound. I will mention it to the dealer at the next oil change. I have feeling they will not hear it or tell me it is normal. Let us know if you get any info.

I get this if my truck is in V4 mode, so I run my truck as O'Brien posted in the city and I have not noticed it anymore.

Sorry for the delay in getting back on this. The more responses this thread got, the more I started thinking I really need start accounting for variables, when it happens and when it doesn't.

What I'm finding is pushing me toward, maybe it is knock after all?

* The biggest constants are two things, 6th gear in V4 mode!
* Following suggestion I drove in Manual (M5 mostly) through slower routes and besides the problem never happening what I didn't realize is the truck NEVER goes in to V4 mode while in manual!?!
* It never happens when the truck is cold (over night like cold) but it also never drops down to 6th and rarely V4 when cold

What my preliminary guess is it's knocking because the 6 speed being geared so low and going into V4 mode (which I think is totally stupid at slow speeds) it's asking too much of the engine at such low RPM and only 4 pistons working and knocks, regardless of fuel.

Someone please share if they know. The Active Fuel Management system does not radically change timing when switching between V4 & V8? Asking because I would think under low RPM high load timing to only 4 pistons on a 5,500 lbs truck would need to be radically different than with all 8 firing?

It really seems V4 shouldn't even kick in until the vehicle's over 45 MPH. Mine's turning 1,300 RPM at ~50 mph, 1,600 at ~65 mph. Without that momentum the engine just struggles?

The knocking or excessive rattle you are hearing at slower speeds in AFM are not normal.Particularly on such a new unit. Mine doesn't react that much. That said,it isn't a perfect sysytem by any means and I too find it quite annoying at city speeds between 35 -55 mph. It isn't in V-4 long enough to do anything but place a burden on the components that actuate it into afm each time when a mere touch of the gas pedal dis engages it. And then the cycle starts all over again. The complexity of the design is destined for inevitible failure. That's why there are several people that either have or intend to simply disable it in an attempt to alleviate this undo load on the engine while it slaves the other 4 along.Not to mention the oil circuit rerouting that's going on through the transition. Hell, GM even provided a way around it at medium speeds by allowing it to not initiate in M5 or below.I figured this little trick out aftter a few days of ownership. Mine will not stay activated for the life of the warranty. It's a personal decision for each of us based on our understanding of it's operation and its probability of causing future issues. Not the least of which could be lubrication related.I reccomend running it in M5 until you drive it over 60 mph and either pop into 6th or shift back to standard auto operation. First thing though ,I'd take it to the dealership with my excessive knock noise issue.