Quote:
Originally Posted by jayno20
I'd still like to hear this sound. Would you classify it as a "rattle" or more like a "shudder"? I know that sounds weird, but I'm just wondering if what I am hearing is for sure drivetrain noise or if it could be this.
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In my experience - it's a rattle. It's not 'felt', at least not significantly.
When I bought the car, last November, the salesman wanted me to test-drive it before we did the paperwork, to make sure it was what I wanted (I had ordered it; I'd driven an A6 CTS-V sedan, but obviously M6 wagons weren't exactly bursting off the lot). It made the noise every time I pulled away from a stop; we both thought "exhaust rattle, no big deal, we'll fix it".
After about a month, and some accumulation of other little issues, I took it back for the fix. They took care of everything except the rattle; I said "you couldn't hear it?"...they said "oh, we can hear it - everybody who drove it did".
One guy thought it was differential, and it does sound like it's coming from the back of the car - when you're in the cabin with the windows up. If you open the driver's door, though, and listen, you can tell it's coming from the engine compartment area.
Anyway, they called GM and were promptly given a TSB that basically says "high performance cars are noisy - it's all normal". I think the TSB was mostly aimed at the fact that T56/TR6060s are just noisy transmissions, but as I said elsewhere I've owned a few and I'm used to that.
So I went to work on it myself. The first thing I found was that the exhaust rattled. I don't know about the ZL1, but the CTS-V has a one-piece system - everything from the front cats back is welded as a single unit. But the angle on the flange on one side was wrong and it rattled in certain conditions. To their credit, the service folks replaced it. And that reduced the rattle - but didn't eliminate it.
Then I found that there was a loose pipe in one of the new mufflers, and I could make it rattle by wiggling the tailpipe. I was *just* able to put a stick down the tailpipe and tack-weld it - but that didn't fix the rattle.
So I went to RPM Transmissions and asked them to tell me what they thought. They were pretty sure it was flywheel noise - these cars have a dual-mass wheel with springs and counterweights, as they put it "there's a lot going on in that bellhousing". And I went to another Cad dealer, a much larger one that sells a lot of V-series. They said basically that they're noisy cars, they'd not noticed my particular complaint but that they had replaced a few noisy flywheels and that your chances of getting one quieter than you started with weren't great.
And that's where I left it until a fellow on LS1tech suggested slipping the blower belt, last month.
I was convinced it was either driveline or exhaust because it sounds EXACTLY like something in the exhaust rattling against the car - metal on metal. It only happens when taking off from a stop, starting when the clutch starts to take up...or when I'm in gear, idling...in a parking lot, say...and I then start to accelerate slowly. It passes through a 'band' of operation where the noise exists, then as the engine leaves that 'band', it vanishes - definitely like a resonance, and it's generally impossible to keep it in that band since the engine has to be loaded. About the only time I can make the noise for more than a split-second is when going up a steep incline - like putting the car on ramps.