I have a 700R4. I had my transmission rebuilt 3 years ago and it has less than 10K on it. A couple of months ago the governor failed and I had to warranty it. Today it started making a weird noise in reverse and overdrive. As far as I know, no shift noise and transmission shifts normally. No slippage, no high RPMs, no hard shifts, etc.
Video of the noise is here:

 

The noise sounds like it is from the bell housing area. Possible cracked flex plate. Or a toque converter bolt is loose. Noise only happens when in gear and there is a load on the motor.

 

Yup, sounds like a cracked flex plate.

VERY EEEEEEEZZZZZZZY to change without pulling the transmission or otherwise doing actual work. Not much $$$ either. Minor aggravation at worst.

 

Ok. One thing I did notice, the noise only happens when my brakes are applied. If I'm costing in gear no noise. Is this normal?

 

Should be free. Transmission is under warranty.

 

Transmission warranty doesn't usually cover the flex plate... it's not part of the transmission, and almost certainly wasn't replaced along with it. Pretty much, never is; it's part of the engine, not the transmission. You can ask but don't expect a miracle.

 

Before I look into that, do you think it could be brake noise? The noise only comes when the brakes are applied.

 

Pretty straightforward to sort the noise into those 2 categories.

If it's brake noise, it'll be related to wheel RPM. If it's flex plate, it'll be related to engine RPM.

 

The only thing that really makes me wonder. Why does the noise only occur when the brakes are applied? When I take my foot off the brake at idle the engine RPM doesn't change but the noise goes way instantly.

I'm not saying it's not the flexplate but it's something to look into as well. It's been a few years since my brakes have been done. Pads are probably 5 years old. Calipers may be 8 years old.
If it is the flexplate, how long is it safe to drive? I don't drive far and I can idle to my parents house if needed. My hours have been cut due to COVID-19 and I'm not sure when I will be able to get it into a shop.

I've also read a few articles that saying the most common cause of flexplate cracking is a misaligned transmission. Does that mean the shop could have messed up when they installed the transmission three years ago? (I don't think they pulled it at all to replace the governor).

 

There's no way "shop" would have misaligned the transmission. There's 2 dowel pins that positively locate it. It only goes in one way.

OTOH, one of The Problems with 70s blocks (I don't know what your motor is), is that the dowel pin drills in the block, aren't centered on the crank centerline. This of course results in the converter being sqeewaw in there, the flex plate having to wobble constantly, and the pump bushing being under all kinds of stress, usually leading to pump whine.

They used to make offset dowel pins for that situation. I seem to recall using .015" ones more than once (just about 1/64" ) to get a manual trans bell housing to align to the crank. Who knows if that's your problem.

But then again, sometimes, flex plates just ... break. It's almost always at the center where they bolt to the crank; the bolt pattern breaks out of the rest of the sheet metal. I've come across more than one that completely severed themselves such that the car had no drive.

 

In case it helps with the flexplate the engine is the original LB9 305 with 120,000 miles. If it helps with the possible braking, car is an original factory 4 wheel disc brake car.

For now since the noise only seems to happen at idle and when the brakes are applied. I'm assuming it's safe to take short trips for now and monitor it closely? I'm hoping I can get it checked out in a couple of weeks.