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What's that sound?!
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Old 09-19-19, 07:48 AM   #1
elbowdonkey
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Default What's that sound?!

I'm the original owner of the car, a 2006 STI with about 75k miles.

It's mostly stock with the exception of:

- Cobb AccessPORT
- Cobb short ram SF intake
- Cobb downpipe
- Southbend stage 1 HD clutch

I need to some tips on how to determine where a new and troubling sound is coming from. I'm happy to record it, but I know how hard it is to pinpoint just from a recording.

The sound happens when I've just started moving forward from a complete stop and at low RPMs (1.5-3k) in first gear. It's a distinct series of knocks - like "knock knock knock knock" and then it's done. Everything sounds fine once I'm moving. The number of knocks varies, but it's usually between 3-10 or so.

I sometimes hear it when letting the clutch out after shifting from 3rd to 2nd as well.

It doesn't happen in neutral, which makes me think it's transmission related.

The only thing I've done is check for loose heat shields, but I'll admit to maybe half-assing that too.

What should I focus on next? Are there tips for ruling out specific causes?
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Old 09-19-19, 12:53 PM   #2
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Default Re: What's that sound?!

No noise doing same in reverse?

Not enough info - but makes me think something moves under torques and something rotational "hits" . Would have to be slow rotation from your description. Would think you'd be able to make it occur then in other gear/times then though . . . so . . .Not enough info . . .
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Old 09-19-19, 03:06 PM   #3
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Default Re: What's that sound?!

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Originally Posted by mheyman View Post
No noise doing same in reverse?
Correct - no noise doing same in reverse.

I also think it happens when something moves under torque, which then leads to rotational hits. Engine mounts, maybe?

As far as info goes, I'll try paying more attention to what happens when I hear the noise and share it here.

One thing I just thought of was that I'd record a video with the noise in the morning. The reason for waiting until the morning: that's when I seem to notice it the most - when the car isn't warmed up.

So there's that too.

I also don't think it's related to the steering angle - meaning, it's not making the sound when the wheels are at any particular position. I can verify that in the morning as well.

Any other things to pay attention that'd prove useful?
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Old 09-20-19, 03:35 AM   #4
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Default Re: What's that sound?!

Do you park on a hill? Something to do with hill assist going off? Then brakes would be rotational.
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Old 09-20-19, 05:26 AM   #5
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Default Re: What's that sound?!

Are you able to describe in general where the sound is coming from? Is it coming from under the center console, from under/forward of your feet, etc? If you're able to describe general location, it may be easier to help locate the source.

For example, I had a similar knock taking off in first gear. Turns out the transmount rubber bushings, being that it was a stock mount, pretty much turned to mush and was allowing for excess movement. Once off the car, I could flex the mount by hand.

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Last edited by viper_crazy; 09-20-19 at 05:56 AM.
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Old 09-20-19, 07:57 AM   #6
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Default Re: What's that sound?!

Quote:
Originally Posted by mheyman View Post
Do you park on a hill? Something to do with hill assist going off? Then brakes would be rotational.
Not much of a hill - a driveway with a bit of slope. Do you know exactly how the hill assist works? I've never even heard of it, at least not on a manual transmission.

There are times when the car has sat for a few days and as I back out it seems like the parking brake sticks, then releases with a more noticeable sound than usual. But that doesn't sound like what you're describing (and rarely happens).

Quote:
Originally Posted by viper_crazy View Post
Are you able to describe in general where the sound is coming from?
Oh yeah. I totally could have done that when I started this thread. This is why I can't have nice things...

When sitting in the driver's seat it seems like the sound is in the center (from left to right), or possibly a bit towards the passenger's side. It sounds like it's about two thirds up in the engine bay, and maybe between the dash and the engine.

I'm working on figuring out a way to mount an old phone inside or underneath the car so I can either confirm or dispute my estimate of where it's coming from. The recordings I took this morning don't help at all - just too much interior and road noise to hear the sounds clearly.

Maybe I can take the hood off, strap my kid to the roof, and have him listen for it?
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Old 09-20-19, 08:02 AM   #7
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Default Re: What's that sound?!

Quote:
Originally Posted by viper_crazy View Post
For example, I had a similar knock taking off in first gear. Turns out the transmount rubber bushings, being that it was a stock mount, pretty much turned to mush and was allowing for excess movement. Once off the car, I could flex the mount by hand.
Do you think you'd be able to tell if they were shot while the transmission in place?

It's an attractive theory since I only recently went beyond stock with the clutch and accessport -if those bushings were old and crusty, adding a bit more torque couldn't have made them less old and crusty.
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Old 09-20-19, 08:16 AM   #8
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Default Re: What's that sound?!

Well, my dude, it seems like we can help narrow it down.

From what you generally described, the pinpoint issue and best guess would be the pitch stop mount, which is connected to the trans and firewall. The factory pit stop is good enough for stock levels. Combine that with a fairly weak transmount, and the problem is rearing its head.

The factory trans mount is designed to lower NVH (noise, vibration, harshness) but due to its design, it means it's fairly flexible. First gear (due to gearing) has the most torque. The trans moves up and down under torque. When letting out the clutch, there is always a bit of chatter that you may likely never feel, unless a bushing is failing or the power has been increased, or both.

You can imagine, when the clutch is doing its slight chatter, the rear end of the trans is basically moving, causing the engine to rock forwards and backwards, which is where the pitch stop mount come in, to prevent excessive movement, but as it seems there's enough movement, the pitch stop is especially using your firewall as a drum, hence the knocking sound taking off in first gear.

Many in this position have recommended replacing the trans and pitch stop with upgraded units. Iirc, Group N is a popular upgrade for decent money.

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Old 09-20-19, 08:18 AM   #9
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Default Re: What's that sound?!

And yes, you could tell it was shot with it in place. In my case, when I crawled under the car, I could literally bench press the trans and see it move. That's how badly worn my trans mount was.

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