1998 C5 Mystery noise when accelerating.

[Alchemy]

There is a rather curious rattling sound that is audible whenever the vehicle accelerates. It seems to do this in all gears (manual 6spd) and is a metallic sort of a noise. We've lifted it, and looked all underneath the car and still can't manage to locate the source. It seems to do this when the car is under load. There are no fault codes, and no other symptoms other than the noise itself. Any ideas?

[leaftye]

Have you done any mods? I believe the MAF descreen causes the car to run lean. Hopefully I'll know firsthand this weekend when I get to check it out with HPTuner.

[99 The Hard Way]

Sounds like it detonating. I got a bad tank of gas once in my 98, and every time I accelerated I got knock. Try some octane boost & run that tank out of gas & try Amoco or BP. Just an idea. Has this just started? Have you fueled up recently? Scott

[Nick Williams]

Could be the check valves in the air tubes going into the exhaust manifolds. Or possibly detonating.

[0rion]

Does it sound like it's coming from the front, middle, or rear of the car?

[Alchemy]

Sometimes it seems as though it's coming from right in the middle, others more from right behind driver side of vehicle. When it was first noticed, it was from just rear of driver side of vehicle. It's sort of a 'clanking' sound.

[Alchemy]

No, no recent fueling. Only use premium (doesn't mean you can't get a tank full of bad gas, though) Coincidentally, right around the discovery of the noise, the fuel gage began going haywire. Not sure if there's a relationship here. Doubtful, the sound sounds too much like something loose or hitting .. ::sigh::... you've all been great-I do ever so appreciate ANY and ALL input on this-

[Alchemy]

No mods recently.

[Alchemy]

Will do. It's not pinging-which would usually be the symptomatic of detonation issues or misfiring, if I am not mistaken.

[ToplessTexan]

Is there any correlation with either vehicle speed or engine speed? Your description makes it sound like it's torque sensitive.

FWIW worth I've been chasing my own vibration/noise issue off an on for a while now. Noticed the other day while under the car that at least one transmission line (A4 car) clamp under the drive tunnel closeout was not properly secured. Fiddling with them changed my sound/vibration. I'm not familiar with the stick setup but the closeout panel does hide whatever is under it...

BTW, the service manual has a decent section in it on noise identification/isolation. Sadly not everybody has a frequency analyzer handy but there's good info there to guide your thinking.



[Modified by ToplessTexan, 8:18 AM 6/17/2004]

[Nick Williams]

Sometimes the ring around the top of the rear o2 sensors will vibrate and make a noise. I chased that for a while after installing headers.

[leaftye]

Maybe it's something else entirely. Try shaking the hood up and down while it's supposed to be latched. The bumpers aren't always adjusted right, and can produce a sound kinda like you're describing.

[0rion]

Have you tried tapping on your cats and see if they're rattling? It sounds like an exhaust issue to me but without hearing it I'm just guessing.

[Dally]

is it through the entire rpm range... do you notice it more on a lower gear or rpm?

I think I have that too... car is getting tuned this thursday .. Ill ask this guy what he thinks... he should know .. he built a 7 sec LS1 67 camaro.

so... i'll let you know what he may think.

[Alchemy]

First off-THANK ALL OF YOU who posted in reply to my question. I CANNOT thank you enough, it was greatly appreciated. The least I could do was update you on the issue; quick update on the noise: the problem HAS been located and erradicated. In the torque tube, on the rear bearing assembly there are three bolts. Well, two were missing. One was located. The other was no longer recognizable as a bolt. It appears that the first one made it past the rear coupler and was 'rolling' around, hence the metallic rattling noise. The second bolt wasn't so lucky. It appears to have been caught up in the coupler, which proceeded to chew its own face. This explains the crescendo that the 'rattling' erupted into about two months ago. The coupler has been replaced, as have the bolts, etc. There's some pics that show the aftermath of it all..http://www.msnusers.com/C5TorqueTube...nw?albumlist=2

Sorry, I couldn't find a place to upload them to on here, and wasn't sure if it was really something appropriate to be uploading onto this site, so please excuse the link.
Thanks again!
-M.

[ToplessTexan]

Thanks for the update!