Author Topic: 67 GT 350 Rattle on Light Acceleration  (Read 885 times)

67350#1242

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67 GT 350 Rattle on Light Acceleration
« on: August 25, 2021, 03:04:13 PM »
I was wondering if anyone else has experienced something like this.

   67 GT 350 4spd - after downshifting to second and going around a corner, as I lightly begin to accelerate I will hear a short 1-2 second rattling sound that appears to be coming from near the passenger side front floor or tunnel area.
  Almost sounds like a loose heat shield would on a modern car - but no such thing is there just a stock exhaust system.
  Would it be possible something loose in the transverse muffler or resonators that could  be transmitted to the front?   I have gone under and banged around on the exhaust and could not replicate.
 Any ideas?
Kurt.
67 GT350  SJ 02/01/67  Gray 4spd A/C
67 Coupe  SJ 11/16/66  White Auto A/C PDB

JD

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Re: 67 GT 350 Rattle on Light Acceleration
« Reply #1 on: August 25, 2021, 03:31:13 PM »
Left and right turns?

Doesn't do it around the corner with constant/steady on the gas pedal, only when reapplying throttle?
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67350#1242

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Re: 67 GT 350 Rattle on Light Acceleration
« Reply #2 on: August 25, 2021, 03:49:06 PM »
Yes both left and right turns.  Only when applying throttle and very short RPM range.  Slow speed corners only,  haven't noticed under constant throttle.
« Last Edit: August 25, 2021, 03:51:54 PM by 67350#1242 »
67 GT350  SJ 02/01/67  Gray 4spd A/C
67 Coupe  SJ 11/16/66  White Auto A/C PDB

T-Bone68

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Re: 67 GT 350 Rattle on Light Acceleration
« Reply #3 on: August 26, 2021, 09:26:56 AM »
Long shot, but check the bolt at the rear of the passenger side front fender, under the car.  I heard something similar after knocking mine loose climbing under the car to mess with the speedometer cable clip at the toe kick there.

67350#1242

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Re: 67 GT 350 Rattle on Light Acceleration
« Reply #4 on: August 26, 2021, 10:10:16 AM »
Just checked the lower fender bolts as suggested - they are tight.   I did notice that when I pound lightly on the rear splash shield there is a similar rattling sound.  I will investigate further - maybe tape it to the inner fender and go for a drive and see if the noise goes away.   Thanks for the tip - hope something simple like that.
Kurt.
67 GT350  SJ 02/01/67  Gray 4spd A/C
67 Coupe  SJ 11/16/66  White Auto A/C PDB

Drew Pojedinec

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Re: 67 GT 350 Rattle on Light Acceleration
« Reply #5 on: August 26, 2021, 11:18:22 AM »
Alternate takeÖ.
Make sure distributor springs arenít too light.
Iíve heard detonation that sounds like a body rattle.

Good luck

67350#1242

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Re: 67 GT 350 Rattle on Light Acceleration
« Reply #6 on: August 26, 2021, 07:13:55 PM »
Quote
Alternate takeÖ.
Make sure distributor springs arenít too light.
Iíve heard detonation that sounds like a body rattle.

Good luck
Good point, sure sounds like not coming from engine bay but may be a harmonic originating  from a detonation.
If so would it occur anytime the car was in a lugging situation?   I could try backing off timing and see if any changes.
67 GT350  SJ 02/01/67  Gray 4spd A/C
67 Coupe  SJ 11/16/66  White Auto A/C PDB

Rukiddin

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Re: 67 GT 350 Rattle on Light Acceleration
« Reply #7 on: August 26, 2021, 07:26:57 PM »
I agree that ignition advance specs can cause the knocking under certain lugging conditions,but todayís fuels are also a source. Old fuel or different source might be something to consider. Verify base timing and change fuel and retest. Good luck

navigator

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Re: 67 GT 350 Rattle on Light Acceleration
« Reply #8 on: September 06, 2021, 01:18:37 AM »
I had exactly this kind of rattling noise and finally find out that there was someting loose in one the resonator (stock exhaust). I could only reproduce the rattling noise with a soft hammer when the exhaust was warm.

Laurent

67350#1242

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Re: 67 GT 350 Rattle on Light Acceleration
« Reply #9 on: September 06, 2021, 07:54:05 AM »
Took the car out yesterday to try and better pinpoint where the rattle was coming from.    First, I discovered that the problem wasn't just on cornering,  but even when going straight at a certain, very narrow RPM range.  I could make it happen going straight and holding accelerator at about 1000 rpm in second gear. Under normal driving conditions I am just accelerating through this range and the noise wasn't noticeable.  The noise was only in a range of about 100-150 RPM and really seemed like the whole exhaust system vibrating but more on the passenger side.

Back in the garage I crawled under there and started tapping around with a rubber hammer - checked all the clamp bolts. etc - nothing seemed amiss.  the only thing I found was that the rearmost rubber insulator on passenger side felt somewhat looser than the driver side.  After pulling it off I could see that the slot in the rubber where hanger passes through was a little wallered out and the hanger could move a little.  I stuffed a small piece of weatherstrip in the gap and reinstalled the hanger.

Boom! no more rattle.  Would have never thought that little looseness way in the back could create the sound I was hearing, but then again it makes sense - the rubber isolators dampen harmonic vibration in the system. 
Really am pleased to have fixed this annoying problem - thanks to all for your ideas.
Kurt.

67 GT350  SJ 02/01/67  Gray 4spd A/C
67 Coupe  SJ 11/16/66  White Auto A/C PDB

greekz

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Re: 67 GT 350 Rattle on Light Acceleration
« Reply #10 on: September 06, 2021, 11:17:37 AM »
Good catch.  Sometimes it is the simplest fix that solves the problem.
SFM 6S1134  '67 GT-350 #2339

JD

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Re: 67 GT 350 Rattle on Light Acceleration
« Reply #11 on: September 06, 2021, 11:57:06 AM »
Glad you were able to locate it and it was an easy "fix".

Thanks too for posting the real problem, certainly will help others.
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