This rattle is driving me nuts, I thought I would ask for help...

My car makes what sounds like a heat shield rattle at 1200-1500rpm or so. It has a HKS Replica exhaust which I suspected to be the culprit at first, so underneath the car I took note of the pipe hitting the W brace and the rear sway bar, and adjusted the hanger so it sat higher, and filed a mm or so off the edge of the resonator (it's the part closest to the W brace). The alarmingly loud clanging noise the exhaust used to make disappeared, but it still rattled. I went under the car again, put zip ties on the hangers to tighten them up a little, filed away a bit of the differential casing where it was close to the pipe, and added exhaust wrap where it would potentially touch something. No difference whatsoever! I'm going to adjust that hanger one more time but I highly doubt it will make any difference.

So here's the thing, it makes the sound in gears 1-4...but not 5 and 6. It rattles even at very light throttle, so it's not because the engine mounts or rear subframe bushings are compressing and moving some part of the exhaust closer to anything else. It's not the exhaust tips hitting the bumper, because there's pretty good clearance and it's a metal on metal sound. It's not gear lash (which I can hear clearly at 2000rpm, a much duller sound), and not a bad DMF (has some mystery lightweight flywheel in it). I checked the cat heat shields, they seem rock solid. I didn't check any other heat shields on the engine, but it seems like those should be rattling in neutral and 6th gear if they were loose.

Any ideas what else this could be? I shift at higher rpm to avoid it, but it's not avoidable when I take off from 1st gear, and it's incredibly embarassing

 

Okay, I came up with one possible explanation that is still a little bizarre: Low MTF level causing a loud gear lash noise? The mesh for 5th and 6th gears sits slightly lower so it would get more lubrication if the transmission didn't have enough fluid.

The Infiniti dealer claimed to have checked the fluid level and said it was okay though. I'll check again when I get a chance.

 

After listening to it again, I think I do still have a little exhaust rattle, but this tinny sound in low gears is still kind of a mystery. It sounds like it comes from the rear, not the transmission...and it's too "thin" to be gears. It's like a wire or thin sheet of metal vibrating against something, only really loud, and not in 6th gear.

 

Are you SURE it's not just the lightweight flywheel?

 

It could be...but no sound in 6th gear despite being the same flywheel and clutch? That's so weird...

One explanation is that the gear lash gets transmitted to the flywheel in lower gears, while the high gears have very little gear lash.

 

Does the sound stop even if you're at a low speed and shift to 6th? Like 40mph you can be in 4th and make the sound but then go to 6th and it stops?

I would look for something in the shifter that's loose as well as checking this "weight plate" thing they bolt to the side of the tranny to help dampen vibration. It's HIGHLY notorious for rusting and falling off and it might be right on the verge of failing and rattling around.

 

Yea, I can have it in 4th gear at 1500rpm, if I step lightly on the pedal, I get rattling, and if I step hard on the pedal, the rattling is pretty loud. It's gone as the engine speed goes up past 1600 or 1700. If I shift it into 6th at say 1200rpm, I can floor the pedal (yea I know, the engine doesn't like it, but just trying to get the maximum amount of vibration) and there's no rattle. I can step on it any amount at any speed with no rattle in 6th, but in 1st gear it'll rattle no matter what even if I'm pressing very lightly on the pedal.

I'll check the dynamic damper thingy, thanks. It might already be gone from my car though lol.

 

I replaced the exhaust with a stock one (well, it was gutted at some point, but anyways it doesn't rattle against the car and is very quiet now), checked transmission fluid, all driveshaft joints, and the mass damper. Now I have a groaning noise at 2000rpm and 4000rpm off throttle (probably the gutted muffler resonating), but the rattle at 1500rpm is still there, so it's definitely coming from the driveline. It sounds like gear lash that's being amplified, so it might just be the flywheel...I have never seen a flywheel make this much noise before so this is very disconcerting. Attached are 3 sound clips I took with my cell phone, first is accelerating in 1st and 2nd gear, where the marble rattling noise clearly overpowers everything else. The second clip I give the engine a few blips (creating the groans), and hold the rpm at 1100 ish, where the grinding noise gets a bit louder than idle. The third clip I rev the engine up to where you can hear a very prominent rattle at around 4000rpm. All of the noise is coming from outside the car.

Is my flywheel really just this loud? Are all single mass flywheels just as bad?

 

Alright I think I have two separate rattles that I conflated.

There's a light rattle coming from the passenger side near the engine, sounds like a rock rolling around, doesn't go away when pushing the clutch pedal. It's very speed sensitive though, 750 rpm it rattles, 700 it's gone, 1100rpm it goes from "knock knock knock" to "knock-knock... ... knock-knock ... ... knock-knock". Sound can be heard at 3000rpm or so, and 4000rpm or so, at very specific speeds. My heat shields are tight... I'm trying to think of what this could be...small piece of cat broken? Timing chain? The weird thing is, it's actually louder from inside the car than outside.

The strangest thing is that I noticed more of this rattling from inside the car at idle only after I changed the exhaust, that seems like too much of a coincidence. The old exhaust wasn't even loud at idle, and I swear it didn't make this sound...

Then the other rattle is when I accelerate in lower gears, sounds like a diesel truck, except much worse. Is that just the flywheel?

 

Rattle 0: Sounds like heat shield rattling at low rpm under load, gets better with higher gears (probably flywheel? I guess I can live with this noise if I just keep the engine speed higher, I give up on this one)

Rattle 1: Pronounced under engine deceleration, very nasty sounding marble grinding noise at 700-1300rpm. This is driving me nuts, I swear it was not this rattly at idle with the louder exhaust on, but there is nothing on this stock exhaust that could make that noise. I noticed that on a cold start, it won't make the noise at 700-750 rpm anymore, but it still rattles at 900rpm. After it's warm, it rattles at idle, but it's still more rattly at certain speeds. Since the exhaust produces a lot of backfire noise, I may try putting on someone's completely unmodified muffler, but I doubt that's it. No metallic ringing that a broken cat would produce when revving it, engine is also dead silent when started.

Rattle 2: Present at 2000-3000, and select engine speeds up to 4000 (after which the engine is so loud it drowns everything out), but only under throttle (blipping it to rev the engine up, no noise on the way down). My guess is timing chain, maybe this is just normal.

 

Can you get a clip from the engine bay with someone else working the throttle.

Rattle 0 definitely sounds like a heat shield. Check the ones that are attached to the CHASSIS directly above the cats.

 

If you can get someone else to drive the vehicle while you stand aside it as they drive past you to replicate the sound helps a ton.

Mine makes quite a bit of noise in the cabin but almost nothing outside.

 

I got a 5 dollar stethoscope from Harbor Freight, and immediately I can hear the passenger side header heat shield lightly tapping against something, so that's one rattle down. Engine just makes normal smooth bearing noises, phew. I wasn't able to get at the bolts with my rudimentary ratchet set, is there any bolt that can't be reached at all without removing the water line?

Rattle #1 is primarily inside the car, it's nearly inaudible outside, but I still think it's coming from the exhaust.

 

The Y pipe is covered in heat shields, I'd start looking there next.

 

Finally got around to getting the car on a lift again, I managed to get rid of almost all the rattling! The heat shield on the chassis above the passenger side cat was a little bent and would just barely come in contact under load, making the light metallic rattle. I bent it so it sits a little further. The other rattling came from the midpipe sagging and hitting the W brace (again...). I bent the pipe upwards slightly and now it's eerily silent.

I still have some noise at low rpm that's more pronounced in lower gears, which I assume is just gear noise. It's still a little loud, but that's true of many noises the engine makes...