06 G35 6MT Coupe with 80k miles here. I've been noticing for a while that when I'm in low RPM/High Load situation there's a noticeable rattle coming from under the hood. This has been on going since I got the car 5 years ago and not something that just showed up.

First thought for me is valve lash but I believe the valve lash is hydraulically adjusted and not a manual adjustment.

Should I pull the heads have them rebuilt with cams and titanium goodies since I have them off or is there an easier fix?

Flame suite on, I've been a Honda dude up until I got this girl. Photo for your viewing pleasure. That's pterodactyl poop on the door...

 

I was about to come and post something about this..

Let me ask you- what fuel do you run, honestly?

I have been running Regular 87 after about the first year I owned my '06 Coupe. Starting about 2 years ago, I'd get a sound like you describe, in the same situation, and only when the engine was cold, or barely lukewarm after sitting for hours. Initially, it was in the RPM range and under load like you describe. Sometime in the past year, it started happening even up a little higher in the RPM range, but still only under load, like 50-90% throttle. I really only noticed it leaving work, because I can accelerate up to 40-60 mph right after leaving the parking lot. Within a minute, or so of running, I can't get the sound to occur, so even in the morning, by the time I leave my subdivision, I can't replicate the noise.

I was suspecting a heat shield, as my sister's 2.5 Altima had almost the same situation, and it turned out a bolt head on a heat shield had stretched out and was no longer properly secured at that one location. However, she recently had what she thought was the same sound return, but the heat shield was fine. Turns out, the dealer identified a programming issue in her ECU, and said the noise was actually engine knock. They reprogrammed it for free (wow!).

On that hunch, I went "Duh.. I'm running 87. Let me try Premium and see what happens." Sure enough, the sound is gone now!

I was at the point I was worried the noise was timing chain / oiling related, since the sound went away so quickly. I dismissed knock/ping because I figured if that was the case, it would certainly be still noticeable, even WORSE, when the engine was warmed up. I'm guessing that perhaps the reason the sound went away was either that the ECU was adjusting timing to control the knock, but only after it existed for long enough to force the timing to be retarded more consistently, or that maybe the fuel ratio map for cold starts was to blame for the sound only existing briefly.

Long story short (TL: DR):

If you aren't running Premium gasoline, try that at your next fill-up, then report back on your results.

 

Thanks for the feedback.

I've actually always ran Premium (93) octane. The rattle that I hear is also when the engine is fully warmed up.

 

How's your oil? Mine makes that sound when it gets about 1/4 qt low.