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Author Topic: Resonance from rear of car  (Read 2582 times)
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Pmustang
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« on: July 02, 2017, 10:31:15 PM »

Hello Folks,

I was having a rather intrusive resonance coming from the back of my 66 350. Not a grinding or a clicking, I struggle to describe it, Kind of like an other worldly resonance. I was positive it was the rear end so I sent it out for a full refurb. New diff, new gears, new bearings (it was checked for wear before I threw money at it and the place is the best in the UK for such work) I now still have the noise but to a lesser extent. It only happens occasionally and is coming from the back end. Sort of like when you shake a sheet of tin, that sort of noise. but a lower tone.

I know, I need to tell you "in gear or out of gear" and will do that next time I have the car out. The side exhausts make such tests a bit difficult.

cheers. Peter

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Bill Collins
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« Reply #1 on: July 03, 2017, 07:34:10 AM »

Peter, this may seem too obvious, but does your car have a fiberglass rear shelf rather than a back seat? I have experienced this condition in my '65 350 and traced it to the shelf vibrating, especially if the spare tire is absent. It produces a low frequency hum that is possibly aggravated the side exhaust.

If you have a back seat, a similar condition can be experienced by the rear seat fiberglass side panels vibrating if they are not attached properly.

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stickshift
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« Reply #2 on: July 03, 2017, 07:47:25 AM »

Peter, this may seem too obvious, but does your car have a fiberglass rear shelf rather than a back seat? I have experienced this condition in my '65 350 and traced it to the shelf vibrating, especially if the spare tire is absent. It produces a low frequency hum that is possibly aggravated the side exhaust.

If you have a back seat, a similar condition can be experienced by the rear seat fiberglass side panels vibrating if they are not attached properly.

This is a truth under certain circumstances. The shelf creates somewhat of a "boom box" effect. Mine was so bad, I filled it full of "construction foam".

This is all very intersting in that my wife claims I am also stone deaf? Huh? I could hear the exhausts?

« Last Edit: July 03, 2017, 07:50:13 AM by stickshift » Logged
OldGuy
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« Reply #3 on: July 03, 2017, 11:01:29 AM »

You didn't say whether you replaced the rear U-joint when you had the rear end refurbished. Sometimes a "dry" (lack of lubrication) and/or badly worn U-joint can cause a resonance (because of the resulting vibration). Like wise a bent drive shaft can be the culprit.

A more likely candidate could be the side exhaust. You could perform a test to see it your issue is caused by the driveline or the exhaust "rumble". Drive the car past the resonance point, take the car out of gear, and shut off the engine. Let the car decelerate through the typical resonance point and see if it goes away. If it does go away, the problem is exhaust related. If it doesn't go away, its drive train-related.

I hope this helps diagnose your problem.

Frank

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Pmustang
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« Reply #4 on: July 03, 2017, 01:46:31 PM »

Thank you guys for your responses

I have over 20 old Mustangs so time spent in any particular one is a bit of a challenge. The noise is greatly reduced now that the rear end was rebuilt so at least we are getting somewhere

No rear seat delete, Have a rear seat, Maybe as mentioned the rear panels. They are quite secure, the car to my knowledge has never been apart and everything seems quite tight. May have to get the wife to drive with me in the back seat

UJs were not changed, they are probably in need of lube if they have grease zerks.

cheers. Peter

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« Reply #5 on: July 03, 2017, 02:49:50 PM »

Driveshaft out of balance? Or a bent axle?

« Last Edit: July 03, 2017, 03:13:22 PM by Coralsnake » Logged

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« Reply #6 on: July 04, 2017, 05:16:58 AM »

rear valance panel loose?

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67350#1242
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« Reply #7 on: July 04, 2017, 06:09:30 AM »

Trap door?   Rear seat latches?

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« Reply #8 on: July 04, 2017, 06:12:10 AM »

Loose nut behind the wheel!    Hysterical

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Pmustang
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« Reply #9 on: July 06, 2017, 11:13:44 PM »

Still a bit baffling

The side exhaust makes hearing it or anything for that matter a bit difficult on application of the right pedal

Its most common going uphill under load. 30-40mph (May be happening at a faster speed but can't hear a thing) I'm thinking its related to the engine causing a vibration. It doesn't seem to happen in a free wheeling (trans in neutral) situation. The fact that rebuilding the whole rear end has lessoned the noise tells me again its vibration related as its much smoother now

Strangely its usually a 2 step noise in quick succession.

That being said CS would be proud of how much use I get out of this car. Its sunny here and I've been carving up the back roads of Essex UK



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stickshift
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« Reply #10 on: July 07, 2017, 06:08:04 AM »

I remember a 65 GT350 that I rode in on the track at the first Dearborn convention (back in the days before the lawyers got involved with track time requirements) that at speed had a rear vibration.

By speed, it was 125 on the bank turns and about 135-140 down the straits.

Bob Aleberto had Eastern Mustang at the time and took the entire thing apart and to my knowledge could not correct the issue.

I can't remember at what speed the "issue" would become apartent but it might have been around 90mph. That is tough to re-create unless you are on a "track". Certainly nothing that can really be done on the street.

So the point is, there are other cars as well that have had these issues.


I'm thinking that because everything in the back was new, that the vibration was mis-identified as coming from the rear of the car. Maybe the exhausts, the clutch/flywheel or even the transmission itself?

Seek out Aleberto. He's around here somewhere. Ask him if he ever fixed Charley Browns 65?

If you can't find him, ask Kopec to intercede. He'll know where Bob is hiding out these days.

« Last Edit: July 07, 2017, 06:17:39 AM by stickshift » Logged
sg66
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« Reply #11 on: July 08, 2017, 08:11:54 PM »

I read this thread earlier in the week and would have placed bets on u-joints or out of balance drive shaft which still may be possible for your car. Now to confuse you, I had my Hipo out tonight and noticed the same type of resonance driving at 50-55 mph in 3rd gear. When I shifted to 4th it went away. I did the same speed in 2nd and nothing but then realized the resonance appeared in 2nd at around 3200 RPM, same RPM as 3rd. It went away by 3500 RPM. For what it's worth, I have 3.89 gears with a locker and had pinion seal and ujoints replaced about 500 miles ago along with rebalancing the driveshaft.

Good luck

« Last Edit: July 09, 2017, 11:32:17 AM by sg66 » Logged
6s1802
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« Reply #12 on: July 08, 2017, 10:12:33 PM »

These things weren't built to be Lincolns. Every time I drive mine I hear a new noise or feel something new. FoMoCo didn't think these things were going to be around 50 years later.

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Pmustang
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« Reply #13 on: July 09, 2017, 12:03:32 AM »

Hello 6S1802. I have twenty to thirty Mustangs at any one time, This noise is not in the normal realm :-)

SG66, I have brand new 389 gears and a traction lock (have to check invoice) Trans is a T5, five speed. So it could be the one bit thats not been replaced, I.E. the driveshaft/U Joints. Your issue sure sounds the same as mine. Are you saying you changed those bits and you still have the noise?
Cheers. Peter

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sg66
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« Reply #14 on: July 09, 2017, 07:20:42 AM »

Peter,

My pinion seal was starting to leak so while I had things apart, I replaced the ujoints and had the driveshaft rebalanced. I never noticed the noise before and if I hadn't read your post, I probably wouldn't have noticed or thought anything about it last night. I have a T5 in mine also but doubt that has anything to do with it. One other thing to consider that we may have in common is solid motor mounts. Mine was built before the cutover, not sure about yours but may also explain why it's not found in your other cars.


« Last Edit: July 09, 2017, 10:12:03 AM by sg66 » Logged
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