Stingray_RSSS

Went for a ride last night with the wife in our 66 coupe. It has a 327/300, I punched it and heard what sounded like something slapping hard under the front of the car, thought maybe I hit something. Pulled off checked it out, didn't see anything hanging, its very clean underneath, having rebuilt the entire chassis earlier this year. I ran it hard again, it sounded like it came from my side under the car, and the wife said it sounded like her side, almost sounds like a belt is slapping hard against the underneath, the belts are tight. I revved it hard sitting idle and no noise, if I just do half throttle there is no noise. Also the car runs good, but have noticed now and then rough idle, which is why I punched it to see if carb was dirty. Its a rebuilt about a year ago. I am going to jack it up and look carefully underneath, but looking for help or suggestions.

Dennis

skids

Bad motor and/or tranny mount?

w1ctc

See if it does it when your alone in the car. The extra weight of another person may cause something to contact the floor.

63C2splitter

That would be the first thing I would check. If it is making the noise under power, a mount could be allowing the drive line to hit the tunnel. Did you notice if the shifter was moving a lot to one side during accelleration? Hope it is something simple.

Dave

vark_wso

If you have power steering, check to see if the pulley on the pump drive is secure.

bopewells

In my 61 with a 5-speed installed, the rear transmission mount bolts loosened just enough to allow the transmission tail=shaft to shift sideways so that under hard (or even moderate) acceleration, the bolt heads on the universal joint were hitting the side of the transmission tunnel. Shifted everything back to center, tightened everything back up, all is well!

rahzip

I agree with motor mount and trans mount.
If the car is a 4 Spd the shifter would raise when you accelerate

Birdball

I agree with the mount, and your fan could be hitting the shroud under acceleration as another noise option. There should be some markes on the inside of the shroud if so....

matt

MikeM

Look at the underside of the driveshaft tunnel in the area of the seat supports. Sometimes the floor sags a little and it hits the driveshaft. Might do it with two people aboard but not one.

66427-450

CHECK THE TIRES.

I experienced a similar noise just before my right front tire came apart and just about tore the fender off.

rongold

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A friend of mine had a 67 that developed the same problem. What happened to his was that the vertical panel behind the seats (it's made of steel) broke loose from its' mounts at each end, and caused the floor to sag with 2 people in the car. The drive shaft was banging against the floor at the transmission end.

Hitch

It sounds to me like the floor pan is hitting the driveshaft back at the rear end. The floor pan typically sags in that area and over time with the weight of two people in the car has made you notice it.. Dave

56Heap

If that is what you find check the rivets on either side of the divider panel behind the seats

MORHPPLZ

maybe a slow moving Hyundai Just a thought

JohnZ

Check the condition of the bushings where the bracket on the nose of the diff bolts to the frame; if they've deteriorated, it'll allow the bracket to rise up enough such that the rear prop shaft flange and U-bolts will hit the top of the tunnel, especially if the floor pan has sagged in that area.

BigT-65

I am getting this same thumping under the car under acceleration with two people in the car. I have gone under the car several times and can't find any places where it looks like there is contact. I have had the differential bushing changed twice over the years and it didn't help. I can feel it while driving and it feels like it is right under the seats. If that vertical support behind the seats is not connected properly where would I look to see it. Under the carpet, under the car? I have been living with this problem for several years and sure would like to get it fixed.

66jack

Both places....

Get under the car and push up and down on floor pan, roughly where the luggage stop would be...look for movement and you will/should see where the rivets should be...if you cannot see it then take up the carpet behind the seat/s at floor level, then you should see where the rivets are or are missing...

MikeM

Not mentioned yet is exhaust grounding where they pass through the crossmember.

Tampa Jerry

Did you use poly or rubber diff. bushings. If you used poly, that could cause the problem you are having. The poly bushings are too thin. Jerry

6T7L71CPE

If it's the rear diff u joint hitting and it's been going on for "years", you should see it coming through your floor hump under the storage area.
Mine had a hole about 2"x3" carved out by this. My luggage stop had popped most of the rivets. These are visible from under the car too.