I've searched through a number of threads, and found one or two that sort of resembled the noise I'm hearing. I've checked most of the things listed in those posts, but haven't been able to track this rattle down. It's not a squeal or screech, but a rattle that's most notable when accelerating. Though, I do hear it at idle now that I'm listening for it.

At first, I was almost positive it was exhaust related (2004, stock cats/H-pipe, 38k mi, Ti mufflers), but I listened under the car with it running and it was fairly quiet. Then, I popped the hood and could clearly hear it. For anyone that's had the exhaust spring rattle on an A4, did the sound travel down and out the sides or work its way back into the engine bay?

I moved on from thinking it was the exhaust to the belts and pulleys. Replaced the main belt with a Gatorback. The noise was still there. Replaced the main tensioner because that pulley rattled and it appeared to have some play side-to-side. Replaced the main idler pulley at the same time. The noise is still there, but maybe slightly less noticeable.

I ran the car without the main belt, and it sounded like the noise was still there but significantly decreased. The only thing I can think of is that it's the harmonic balancer/dampener making the noise since that spins with and without the main belt on there. Maybe when the main belt is on it's putting more strain on the balancer? There doesn't appear to be any wobble, and I painted a white line on it to check if the hubs are spinning independently. After a couple of days of driving it's still lined up.

Any other ideas? I suppose it could be in the A/C system, but wouldn't the noise stay constant with the main belt off?

 

Just a follow up for anyone searching C5 noises.

Turns out it's the alternator.

I took it over to my local Vette tech and went through the car with him. Ran it with the belts on & off, and A/C on and off. Also checked out the harmonic balancer and power steering pump/pulley. Put it on the lift and examined the exhaust (cats, hanger springs, etc.).

He hooked up the Tech 2, kept it at a constant 1500 RPM and when he unplugged the alternator the sound went away. It's charging fine, but being noisy. I might go ahead and swap in another one because I'm getting tired of this noise.

 

Be VERY careful in selecting a replacement alternator and DO NOT turn in your old one until you have a good functioning replacement. Your car being a 2004 with an auto takes a 145 amp one with a decouple pulley.

The C5 takes a unique alternator unlike other GM ones that look identical. If not correct you will get a Charging System Fault message.

Here is some info on this from Evil-Twin here on the Forum who is an expert on this issue and was an engineer on the C5 engine design team.

The L-terminal circuit from the generator is a discrete circuit (a discrete circuit has no splices and only one source and destination) into the PCM. The PCM applies ignition voltage to the generator L-terminal circuit. A small amount of current flows from this circuit through the generator windings to ground to create a magnetic field which starts the generator process. When the generator is at operating speed and producing voltage, a solid state switch for the L-terminal circuit in the generator opens and the PCM detects that the initial startup current flow has stopped.

The PCM expects to detect low voltage on the L-terminal circuit prior to the generator rotating at operating speed and conversely expects the circuit to be at ignition voltage potential when the generator is operational. When the PCM detects a fault (circuit shorted to ground, or circuit shorted to voltage), the Driver Information Center will display Charging System Fault.

The generator has an input to the PCM called the F Terminal to indicate the percentage of total capacity that the generator is producing. This signal is detected by the PCM as a duty cycle from the generator and displayed on the scan tool as a percentage. The PCM can monitor the generators output under all conditions to determine if it is functioning normally.

When there is low demand from the electrical system on the generator, a low duty cycle percentage will be displayed. As more accessory load is placed on the generator, the duty cycle output detected by the PCM will approach 100 percent. A normally functioning generating system will never reach 100 percent as indicated on the scan tool.

The L and F terminals are the red and grey.

PS: Many people rebuild their alternators to avoid replacement issues

 

Thanks for posting that dadaroo.

I've been combing through those posts by ET & BC this morning regarding the charging system in the C5. That's definitely something I'll have to keep in mind. I found what looks like a direct replacement on Rock Auto, but I'm still nervous about replacing my original with anything else.

Rebuilding it might be the way to go.

 

If I do go the route of purchasing a new/"new" alternator, what's the difference in these two parts?

AC Delco: 10353440
AC Delco: 3341493 (also uses 19134486)

From what I can tell, the 3341493 unit is a reman and the 10353440 is a new O.E. piece. I imagine that the 10353440 is preferred if not rebuilding the alternator currently on the car? This is on a 2004 A4, btw.

 

I am working on a list of P/Ns for the 2002-2004 years and it is very confusing. However it does seem from various sources (and GM dealer) so for that the 10353440 is correct for 2004 and has the decoupler. I have 2 NOS 145 amp Valeo without decouplers at one of my warehouses I plan to look at also. Bought them right after I got the car not realizing I needed a 110 amp version.

Decouplers and regular pulleys can be swapped just for your info.

 

Just wanted to update this thread in case anyone searches rattle-related posts in the future.

I went ahead and replaced my alternator with the #10353440 ('04 A4), and had no issues with charging system errors. It also eliminated the noise that I was hearing at idle from the front of the motor.

However, I still have what sounds like a rattle while accelerating starting around 1100 RPM and up. I suppose it could be the infamous exhaust jingle (marbles in a can, etc.) that pops up on these cars, though, I haven't listened closely to another C5 to see how my noise compares. The reply in this thread mirrors what I'm experiencing. I searched around, but didn't find a follow up to see if he swapped in an X-pipe to eliminate the noise.

I'll have to keep a close eye on speed, RPM, drive vs park vs neutral as it relates to this sound to further investigate the cause.

 

I finally got around to recording some audio since that's probably better than any description.



The first part of the clip is with the car in Park and AC is on (it also makes the noise with the AC off). It seems to be a constant rattle as the throttle is held steady (around 1100 RPM).

The second part is with the vehicle in Drive (foot on brake) and AC on, and I was able to get the noise to really come through. It's exaggerated because I was able to keep the throttle pretty steady (again around 1100 RPM) for a while.

Prior to this, I tried turning the AC off and on while driving home, but it made no difference. TIA to anyone who takes the time to listen.

My thoughts:
-Bad cat internally
-Broken welds between cats
-Loose 02 sensor(s)
-Loose heat shield(s)
-H-pipe

I replaced the main tensioner and idler pulley, as well as the alternator. Doing that eliminated the other noise that I heard at idle. I thought they were one and the same, but obviously not.

Is this what everyone describes as the "exhaust jingle?"

Car is a 2004 A4 (40k mi) with Halltech Stinger intake & Ti exhaust as the only mods. It made this noise prior to the Ti mufflers going on, and at the time I hadn't spent any time tracking the rattle and thought it might be in the stock mufflers. Guess I was wrong on that.

 

I'm going to pull the A/C belt this weekend to definitively rule the belt and pulleys out.

I've paid close attention to the noise every time I drive & it's only happening when I'm applying throttle and starts around 1000 RPM.

I can drive around under 1000 RPM, and everything is good. As soon as I hit that the noise kicks in and stops the instant I take my foot off the gas pedal.

To me, that seems more likely to be exhaust related.

The car is not throwing any codes and runs fine otherwise.

 

Finally got this resolved.

Went back to square one, and used Evil-Twin's guide for checking the belt systems. Ran the car with the A/C belt off and main drive belt on since replacing the alternator had stopped the noise I was hearing at idle from inside the vehicle. I figured that system was good to go.

Turns out the other noise wasn't happening any longer. Picked up a new A/C idler pulley, tensioner and belt. Replaced them this afternoon, and the car is as quiet as ever.

So, if anyone has what sounds like an exhaust rattle starting around 1000/1100 RPM check those pulleys!

 

I have a similar Vette...Which pulleys did you use? The oem plastic ones from GM or aftermart metal ones? BTW...great info...thanks.

 

I used the Dayco replacement pulleys from Advance Auto, and I believe they're steel. The only thing is that if you want to reuse the stock dust shields, you'll have to add a spacer of some sort or the pulley bearing will rub. I used a washer and didn't have any issues. The bearings are sealed and don't necessarily need the shields, but I felt better throwing them back on.

 

Nice write up... you are persistent.