2013, 5.3L, I notice at low speeds, say 15 - 30 mph a rattle/slight vibration (fur lack of better words). My foot will be off the gas pedal and when I go to accelerate, a brief, 1 or 2 second rattle. I have put the dashboard on the screen where it show V4 or V8 mode and from what i can tell, it occurs during the switch from V4 to V8. This is the first vehicle I've owned with this feature. Is it the norm? I noticed it as soon as I bought the Ava. Even when I'm at a dead stop sometimes I will feel the rattle/vibrate in the engine (what it seems like to me). Notice it alot in 25mph or 35mph zones.

 

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I have the same thing with my 2013 I took it to the dealer they said that's normal. I bought one of those AFM delete modules that plugs in below the drivers side dash and it keeps the engine in V8 mode no more aggravation.

 

I have the same truck. The rattle may not be related to the AFM because you indicated it happens when stopped. The engine reverts to V8 mode when stopped so it may not be any different than non AFM vehicle. I would listen at the exhaust system possibly a foreign object in it or a broken or loose baffle inside the exhaust system. My truck does not do this hence it is not "normal".
I do hear a slight "purring" if you may, sometimes at highway speeds in 4 cylinder mode - in my opinion that is normal. The AFM saves fuel so I'm not keen on disabling it just yet.