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Discussion Starter #1
It's a 1996 Thunderbird 4.6L and 4R70W. Here's the symptoms:

At low city speeds, like 60 km/h, at very light throttle, there's a faint whirry/humming sound. I can almost make it happen on and off at will by pressing the throttle very lightly and releasing it.

Also, during certain city speeds and upshifts, or if I feather the throttle like I described above, I can often produce subtle but noticeable "bonk" or "dunk" sounds.

Third, on the highway, at 100km/h and above, the whole car vibrates noticeably. I can feel it everywhere, steering wheel, floor, even my face. Seriously, it's ridiculous.

This has been going on for years. I think I've eliminated three possible causes.
I went from my stock driveshaft to a Dynotech from Evanoff to another stock driveshaft from another car. Problem still there. I also swapped on a friend's wheels and tires and went for a test drive. Problem still there. Last winter I had a new engine built and installed. Problem still there.
I also once jacked up the rear of the car and spun up the drivetrain with the rear wheels spinning. Noise and vibration were there with this setup too.

About ten years ago I had some upgrades done to the transmission like outlined in Jerry W's paper, and I had an Auburn Pro differential installed. Car ran fine for years after that. Anyone got any ideas?
 

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The Band, not the Disease!
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U-Joints? Or else, see if there is something wrapped around your driveshaft, IE like a plastic bag, or wire, or something of that nature. Do you have a CLICKING sound when you go from, say, reverse to drive?
 

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Discussion Starter #4
There's nothing on the driveshaft, definitely. About 11 years ago I had to have my original driveshaft replaced under warranty because of the forward/reverse clunk/snap sound probably caused by bad u-joints. The stock replacement also developed the same clunk after 3 years, but I lived with it for a while and the vibration started happening too.
The Dynotech driveshaft cured the forward/reverse clunk/snap, but the highway vibration was still there.
The torque converter is a Mark 8, installed about 6 years ago.
 

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Discussion Starter #6
The driveshaft and u-joints on the car right now are from another 1996 bird, but like I said, it's the third driveshaft I've had with these problems, so process of elimination would suggest it's not the driveshaft or u-joints.
 

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The Band, not the Disease!
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Well if you know the ujoints are good, i would check suspension components and maybe your axles. I would put the car on 4 jack stands and check everything. Check upper and lower ball joints, wheel bearings, tie rods and ends, etc. Remember: There needs to be a slight load on the suspension when you check the ball joints... IE put a hydraulic jack under the Lowers and move the wheel with your hands. If not take it to a shop.
 

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i had a vibrating noise also, i found out the rear upper control arm bushings were bad. jack up the back of the car and grab the wheel and see if there is any play or movement
 

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Discussion Starter #10
I left the car with a good transmission specialist yesterday. He drove the car and confirmed the noises and vibrations I mentioned. He's confident that the whirry hum at low speed is gear noise, but didn't want to speculate on the other items.
 
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