I've had this problem ever since I aquired my Dart, it's a sound in reverse when it's cold. After the car's driven for a while if I go in reverse it'll be gone. My transmission feels fine, the only odd occurance I've ever had was one time I backed up and my tires hit a curb, no big deal except for when I put it in Drive it didn't move and made a horrible noise. That was last year and the problem hasn't happened since and normally doesn't have any problems. When the car is cold in reverse the sound sounds almost like what I imagine an electric motor would sound like, a sort of humming noise, in drive even cold this noise is not present. Since this problem doesn't seem to be causing anything I was just curious if maybe there was some sort of simple fix or if it's maybe a sign of something that I should start thinking about. And if anyone can possibly explain last year's event that would be great(I've never know a car to fix itself yet that day it did). Oh, and my rear axle is a posi-traction unit and the 7.25 inch size. Automatic three speed transmission.

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My car may be slow to sixty, but at least it's been reliably slow to sixty for more than thirty years.

A 7&1/4" Sure Grip!

Really!

It's possible that when you hit that curb, you also displaced the exhaust system, so that when you shift into reverse, the engine and trans torque over, moving the exhaust over till it contacts the floor, or frame, or a bolt and transmits that vibration throughout the car, causing the noise. In drive the engine and trans would torque over the other way, bringing the exhaust away from whatever it's contacting. As for the sound disappearing when the engine warms up, remember when cold the engine is on high idle, and would twist the motor more than when it is warm and the curb idle speed drops.

As for why you were freewheeling and grinding in drive that one time, I have no idea, how hard did you hit this curb exactly??? Did it rattle your fillings out of your teeth?

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

No, I didn't hit the curb hard. Is the small sure-grip unusual? Or was the second part of it wrong.... It's 7 something suregrip at least, I'll check my manual to see what the exact size is.

The backup noise didn't appear after hitting the curb, though I guess it's still possible though my exhaust is on well. I just back up to the curb at 5mph or less(no flying into it like a speed demon to see how high I could make my car jump O.

My transmission(with me at least, only have owned it for two years though) has never seen any major trauma outside of a couple of large pot-holes Spokane's known for, and that would be more like to effect the front than the back since I mostly avoid them and before the back hits them.

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My car may be slow to sixty, but at least it's been reliably slow to sixty for more than thirty years.

Yes the 7&1/4" SG unit is very rare. Take care of it!

Without hearing the car Im going to stick with the exhaust theory. Still empty on the trans problem. Take some pressure readings while it's acting up and see how it compares to spec.

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

How do you take pressure readings? Luckily the problem has only happened once and I never have a chance to take the pressure reading when it's screwed up but if it ever does happen again it'd be good to know how to check. When it gets warmer I'll check the underside of my car with it in reverse(and the axle on jackstands.... the last thing I need is to die a horrible death out of peer stupidity) and see if anything's happening with the exhaust. Err... I don't get the weather. Two days ago it was almost fifty and yesterday it snowed.... then melted... then snowed, and today it hailed, rained, and had about an hour of sunnyish weather. Yet politicians say our pot-holes are attributed to studded snow tires, not the thaw cycle mixed with the problem of a bad grade of asphalt.

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My car may be slow to sixty, but at least it's been reliably slow to sixty for more than thirty years.

I posted this elsewhere a few times... there are a few pipe plugs on the side of your transmission that you can tap into with a high pressure hose with a 1/8" NPT end on it. Put a 0-300 PSI gauge on the end. Your manual will tell you where the ports are, what they should read and when, and what each port measures. (ie lubrication pressure, converter pressure, forward packs, reverse pack and so on).

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

Thanks!