First let me preface this request for information with an emphasis on the term "noise", and not "gear".

In low RPMs (~800, above idle, below cruise) my '89 automatic will make a rattling noise that is sounds most like gear grinding in a manual, however the noise softer and quieter than grinding gears in a manual transmission. It is a metallic chattering, I'm sure most of you can guess at what I'm talking about.

Some additional information:
1. The transmission shifts smoothly through gears, and is very well behaved. It exhibits none of the common signs of failure that I am familiar with.
2. I have checked ATF levels and they are within acceptable parameters.
3. The car is leaking small amounts of ATF. (Do ATF stop-leak products work? I have a bottle of Lucas Oil ATF conditioner.)
4. Probably unrelated, the car is leaking oil from the rear china wall (I'll be fixing that soon, jeebus help me I've never done anything that complicated before, FSM is on the way)
5. Probably unrelated, backing up over uneven surfaces causes the ABS to engage, I've never had a car do that before.

So, any thoughts as to what might be going on here?

An anecdote, my Trans Am had a rattle when I first ran it. The catalytic converter had been damaged, probably due to ground clearance issues, and the element would rattle at low RPMs (idle) but would stop when driving. My hope is that a pre-cat didn't make it over a speed bump, since it's cheaper than transmission repair.

Thanks for reading my long-winded post.

 

You could have interference between the torque converter converter cover and rotating stuff inside. Remove it and see if that solves the noise problem (contact spots will be obvious when you take it off). I'm 99.99% sure you need to remove your starter to get the cover off... Mine had a bent tab near the starter that the flywheel was rubbing against. I can get my cover off by pushing the cooler lines out of the way.

Where is the trans leaking from? It could be as simple as a bad seal that can be easily replaced. Do you have a cork pan gasket (guaranteed to leak)? Fill pipe seal? TV cable seal? Front pump seal? Rear output shaft seal?

The ABS will engage if it sees differences in the rotating speeds of the various wheels. I don't know how much of a difference it takes. It doesn't care whether you're going forward or backward -- it's just counting pulses from the wheel sensors.

 

I'd have to take the car to a mechanic to service the transmission, I'm not familiar with that kind of work.

It appears that the leak is coming from the pan gasket, but I'll need to clean it up before I can tell.

I've had the car for a whole two weeks at this point, and these issues are disappointing. My Trans Am is in worse shape cosmetically and is less problematic at the same time.

 

I just had what sounds like the exact same problem fixed on my 87. There was a bearing in the torque converter coming apart. Had to have the converter replaced. They also replaced the pump in the transmission

 

I'd check the flexplate cover contacting the flexplate or exhaust contacting frame. Catalytic converter rattle?

 

Yes, my Trans Am has its cat located right behind the front wheel well, a lower clearance that the Corvette and SoCal has plenty of uneven parking lots. I suspect the previous owner smacked the cat on a speed bump and shattered the element. The exhaust shop that replaced my cat confirmed my theory and the car started passing emissions.

I'll check the transmission clearances as suggested.