Engine rattle

[StephenW]

So I posted this before as a muffler rattle - but then I ruled that out after replacing the muffler so new thread.

I get a rattle (which I thought was the muffler) with the symptom listed below.

Ok. Back to drawing board on symptoms.

Not speed dependent but appears to be rpm dependent.

Can get it to come in at 40 mph and 70 mph.

Key seems to be rpm - 1500 seems to be the sweet spot to bring it in but anywhere from 1250-2000 and it comes in. Acceleration only, no deceleration. Not in first, second or third. If you hard accelerate from stop it does not come in.

Cruise control really amplifies it. Cruise make engine sit right around 1500 and it comes and goes, comes and goes.

Sounds like a piece of metal vibrating as air flow goes by. Like it is resonating.

AFM does not seem to play into this. Does not happen when shifting to/from V4/V8.

I appreciate any theories.


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[iamdub]

Don't these things have a flapper in the exhaust system that is known to start rattling over time?

 

[wjburken]

Sounds like a resonance thing. Have you checked all your heat shielding underneath to make sure it’s tight and not hitting anything? All the hangers are tight?

 

[StephenW]

Sounds like a resonance thing. Have you checked all your heat shielding underneath to make sure it’s tight and not hitting anything? All the hangers are tight?

I was under the car on a lift with two muffler techs and a mallet. First they thought it was rocks on the heat shield but disproved they.

Only thing that seemed even remotely possible was a little bit of noise from the flex joint when struck by a mallet but not expected to be heard in the cabin.

Are you talking about an issue with the resonator itself? After the muffler?


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[wjburken]

I was under the car on a lift with two muffler techs and a mallet. First they thought it was rocks on the heat shield but disproved they.

Only thing that seemed even remotely possible was a little bit of noise from the flex joint when struck by a mallet but not expected to be heard in the cabin.

Are you talking about an issue with the resonator itself? After the muffler?


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I was thinking more of external to exhaust system. That being said, my dad’s truck had a catalytic converter that liked to rattle at idle but when you gave it some gas, it stopped. Resonant frequency’s can do weird things.

 

[StephenW]

It also does not come in if the car is in park and the engine is rev’d through the offending rpms.


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[swathdiver]

It also does not come in if the car is in park and the engine is rev’d through the offending rpms.


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I bet it's airflow and engine movement dependent. Their are little heat shields all over the place, from the gas tank to the motor mounts and headers and including some on the firewall itself. Check them all and then check them again.

 

[StephenW]

I bet it's airflow and engine movement dependent. Their are little heat shields all over the place, from the gas tank to the motor mounts and headers and including some on the firewall itself. Check them all and then check them again.

Yeah I was doing that with the muffler tech.

But if it was that wouldn’t it be speed dependent and not rpm dependent?


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[swathdiver]

Yeah I was doing that with the muffler tech.

But if it was that wouldn’t it be speed dependent and not rpm dependent?


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Well, at certain rpms the motor is at a certain speed usually. @Doubeleive Had a mystery rattle last year or so, what was it Wes?

 

[wjburken]

It also does not come in if the car is in park and the engine is rev’d through the offending rpms.


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I bet it's airflow and engine movement dependent. Their are little heat shields all over the place, from the gas tank to the motor mounts and headers and including some on the firewall itself. Check them all and then check them again.

Agree with @swathdiver. When the vehicle is idle, you don’t get the induced vibrations from the transmission, drive shafts, axles and wheels. As I said before, vibration is a screwy thing and can drive you nuts.