I have an 86 Fiero GT. It has almost never been driven (9000 miles). So it has had many minor issues that I have gone through and fixed,… but there is one last issue. To begin with it started making a pulsing wha wha wha sound that was very faint. So faint that almost nobody could hear it but me. Now there is a rattling noise that go’s along with it. I have located where the rattling noise is coming from. There is a heat shield on the alternator that rattles in this pulsing manner. The heat shield is on there just as tight as it can be. Every time that I ask a mechanic they tell me that the heat shield is loose. I then tell them “no I checked that and its on there tight”. I then ask "is it possible that the pulley on the alternator, or an other pulley is out of alignment? Or is there a bearing in there that could be bad? The answer is always the same “oh no,… definitely not”. I can’t help but think that I am right about this. Also I been told (and I can’t blame them) that mechanics really hate working on these cars. So I wonder if they are just trying to get rid of me. What do you think?
Maybe the noise travels and the heatshield resonates?
Sears sells these handy automotive stethoscopes that may help you determine where the noise really comes from.
Fieros really are special interest cars. They are a bit quirky but there’s a fair following out there. I see them at vintage car shows a lot lately.
You may want to attach yourself to a club that specialize in it - maybe they can direct you to mechanics that know about them.
On my Fiero…that I flipped over…the heat shields on the exhaust would resonate just like and at the same speed as any other shield on the car…if you think its the alternator and it isnt…start looking near your exhaust…and I believe there is one between the engine and the driver compartment also… look all over cause that car has shields aplenty… they will all rattle in synch so search around.
Does the car have to be moving for this to happen?
No. It does not have to be moving. It happens at about 2500-3500 RPMs, and presumably after 3500 RPMs, its just that as the RPMs go up, the the gap between the noises’ becomes shorter until they seem to blend in to one constant noise. I wish I could explain it better. Also just to be clear, it is not the rattling heat shield on the alternator that is the problem. It is what ever is causing the alternator to vibrate that is the problem. As I said, in the beginning the alternator was not vibrating enough for the heat shield to rattle, so I could not locate where the sound was coming from. At first it sounded like there was some sort gas (like air or something) was being released each time (perhaps “wha wha wha” is not the best way to spell this sound).
I wonder if there’s a break in an exhaust gasket somewhere…
Whichever shield is vibrating is doing so because of the harmonic wavelengths produced by the running engine -vs- the free length of the shield(s).
Once you narrow down the culprit shield, stick on some heat tape or other insulator to change the vibration characteristics. Or space permitting , add a bracket to hold a different position of the shield.
harmonic vibration ?
Have you ever tuned a guitar or ridden in a twin engine propeller airplane ?
The two strings being tuned to each other and the two plane props each produce sound waves and when they’re slightly off from their twin a wavy pulsating sound is heard until the two match exactly.
The shield can’t coordinate it’s vibration with the engine’s so it becomes audible.
There might be some sort of calculus type answer for this such as a change to the shape of the shield to sync vibration rates. Should the sheild be shortened. lenghtened, widened or narrowed ?
Sure a donut gasket on the exhaust could have let go…check into that also for sure.