My BRZ rattles while rolling in neutral (video)
 

My manual transmission 2014 BRZ makes a rattling/flapping noise sometimes between 5mph and 30mph when I am rolling in neutral. The most difficult thing about this noise is that it seems to happen randomly, sometimes it happens and sometimes it won't happen. Here is what I've noticed:

The noise sometimes happens while I'm braking
The noise sometimes happens when I'm NOT braking
The noise sometimes does not happen at all.
The noise only happens in neutral at idle RPM
The noise happens while driving straight, not turning
The noise sounds like it's coming from under the hood in front of the driver
The noise started happening around 4000 miles on the odometer
The noise started happening once summer began and I used the A/C more often
I've never heard the noise happen when the A/C is turned OFF (I need to test this more)
The idle RPM seems fine when the noise happens (tachometer reads ~600rpm)
The shift knob does NOT rattle with the noise
The car doesn't shake with the noise

Here is a video of the rattling (you will need to turn up your volume to hear it).
[ame="https://www.youtube.com/watch?v=xWO3pzi1ICU"]Manual BRZ flapping rattling noise while rolling in idle - YouTube[/ame]

I suspect that the rattling is coming from the compressor for the A/C turning on while the car is rolling in neutral at idle rpm. Maybe the compressor lags on the engine and causes that flapping noise. The workers at my Subaru dealership couldn't figure out what was causing this noise. However, one worker who rode with me thinks the noise does not sound mechanical, rather it sounds like something is flapping around. We agreed that I would check back with them later, and if the noise continues or gets worse, we would test drive a BRZ on their lot to see if other BRZ's make that noise. I have made no changes to my BRZ besides a K&N air filter and mudflaps, but I'm very sure neither of those things are causing the noise.

My questions are:
Does anyone else's BRZ/FRS make this rattling noise?
Is this rattling noise a symptom of a serious problem?
Can this rattling be ignored, like we ignore the idle chirping/cricket noise?
Do you know what is causing this noise?

Thanks

hmmmm.......just curious ..... why are you "rolling" in neutral..??

Does it do it with the clutch in & out or just one way .. ??

As far as your questions, I's say:

1 - mine doesn't make this noise (of course I'm old and half deaf)
2 - I doubt it
3 - oh, yes ....especially if your half deaf
4 - sorry, I have no idea

I hope you get it figured out ......:)


humfrz

Thanks for your response. I'm rolling in neutral because of my driving technique with stick. For example, if there is a red light ahead, then to slow down I will put the car in neutral and apply the brakes (the other method would be do downshift through each gear as you slow down, but I prefer to just leave it in neutral). I don't leave my foot on the clutch while the car is rolling in neutral, I remove my foot from the pedal completely once the car is in neutral.

I also doubt that the rattling noise is a serious problem. But I'm a little worried because I don't know what the noise is and I haven't found anyone else who has the same problem.

I'm apparently too old, because I couldn't hear it :(

Sorry, the video is very quiet because my dashcam doesn't record audio very well. But I can hear it when I turn my speakers very loud.

The best way I can describe the noise is it's like when you accidentally lug the engine from letting the RPMs drop too low while the car is in gear. Except when I hear my BRZ rattle like that, the BRZ is in neutral and the RPMs don't seem to be dropping low at all (staying around 600rpm). Furthermore, neither the car or shift knob rattles when the noise happens.

I've experienced the same thing when coasting in neutral. I don't have a solution, nor am I a mechanic, but I have wondered if the car idles lower than it should. I've seen the rpms drop as low as 400 and then sputter back up to a decent idle of 6-700. I'd be curious to hear what folks with more car experience have to say.

I will hear this when revs are low & I'm creeping at a slow pace with a lite foot on the accelerator pedal - it's that noisy gearbox. I just changed my gearbox oil & it's much quieter now, shift is still ordinary when cold but definitely quieter !

No need to roll in neutral from those speeds. You'll save gas by leaving it in gear when coming to stoplights, just downshift to the appropriate gear and clutch in when your rpm's reach 1100 rpms.

There's no need to downshift through every gear coming to a stoplight.

And you're clutching in like a 100 feet before the light. Is that your normal driving style? You can clutch in much closer to your braking point, just get your timing right.

I usually downshift one gear, but sometimes maybe 2 gears if I'm in too high of a gear. And try and roll as slowly as I can towards red lights. This saves gas over approaching an intersection as fast as you can and then making your car idle in neutral, or rolling as you're doing it.

I've been driving stick for 6 years and I always thought it saved gas by going into neutral instead of leaving the car in gear. But now that I think about it, if the car is in gear while rolling, then the rolling of the wheels is what keeps the engine revving and perhaps this uses zero gas. On the other hand, if the car is in neutral while rolling, then the car is using a little bit of gas to keep the engine revving at idle RPM. I also found this article that supports your claim: http://www.popularmechanics.com/cars...l-fuel-economy


Thank you for enlightening me.

When I run A/C and the compressor kicks in at idle the RPMs will dip a little and it does make the feeling of engine lugging. I believe it's completely normal, it's just spinning up the compressor from a dead stop at idle RPMs where there's little available torque. If you want to see if that's what's causing your exact noise, warm up the car fully (so the RPMs will idle normally since they idle higher when cold), park, and hit the A/C switch and turn the temp all the way down to force the compressor on. See if that replicates your sound.

And yes, when you decelerate it's deceleration fuel cut that triggers. If you ever have an OBD2 app running while driving you'll notice AFRs shoot to the top of the scale since there's no fuel being injected and the momentum of the car keeps the engine spinning.

Mine does this as well. I read somewhere else on the forum that it's the flywheel rattling when the RPMs drop too low. I asked on the scion site if it will damage the flywheel but no response yet.

Also, since I first noticed it, it has been getting louder and more frequent..

I have heard many noises come out of my car due to mods etc but I have not heard that exact one.

Our stock flywheels are damn near silent - although, mine is aftermarket and that is exactly the RPM range where it does chatter. My chatter is a higher pitch/frequency though.

Mine does this exact thing too, I never made the connection that the A/C condensor could be responsible. Ive noticed it for a few weeks now and I always chalked it up to flywheel noise or some other driveline related noise. I noticed with mine though that the noise will occur more frequently when in neutral and slow/brake quickly to come to a stop. If I coast for a while and come to a stop slowly and smoothly, the noise almost never occurs. Also, when the noise happens my rpm needle dips a couple hundred rpms, so it must be at least indirectly related to something running off the pulleys.