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Just started having this issue out of nowhere with my '98 automatic daily. Already checked the fluid levels and they are fine, as always.

Started her up one morning, the only thing I can think that was different was it just dropped below freezing that night for the first time this year, and it was making this... Grinding ice noise. It's probably metallic, obviously, but ice in a blender is more what it sounds like. Happens on and off in idle, and low gears but once I start moving as long as im above 10mph even in first gear, it goes away (or at least you can't hear it over the engine/flowmasters). Happens in reverse as well, but I don't seem to notice it in neutral as often (though it does happen). I'm not particularly hard on her as this is only my daily, and being an unmodified 3.8 other than the exhaust it's not as though I could blow it up simply by accelerating onto the highway. To that effect, I should also mention that for the last 4 months or so it's stayed inside the city where I live and I haven't taken her on any large trips of any notable distance. Driving only the .3'rd of a mile or whatever to work and back, with the occasional wal-mart trip which is just across the street.

Did some research on what I thought it was and it would appear that it's a bad bearing in the transmission, but I don't have any feedback at all through my shifter or... anything at all, other than the noise. There is no whining or any other noises at all like i've seen in other cars when the bearings go bad, but that may just be my particular car. Not sure a better way to confirm the diagnosis other than tearing it apart, and I am trying to avoid that for now. There is a reason it's a daily.

Of course i'm not taking it to a mechanic or i'm sure my transmission will be "on the brink of destruction" regardless, i'll just tear it apart myself if it comes to that. Just looking for someone who may have experienced this noise and can confirm or add on to what I already suspect is the case.

Ill upload a video later of the noise so you can hear it more clearly than I described. There is no abnormal shaking or vibration, just what sounds more like a loose hangar or something in idle. <It's not that, but it doesn't sound like "grinding" grinding either. Softer? Not sure, again might just by this vehicle i've never heard it happen to before but it sounds a bit odd.
 

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Torque converter!

Once you get moving the converter impeller and turbine shaft reach near identical speeds but at idle in any range (R/D/3/2/L) the turbine shaft aka input drum shaft, is still but the converter is spinning. Inside the converter are a bazzilion "vanes" that look like a meat grinder. I believe whats happened is one of the fins "vanes" has come loose and is being slung around inside the converter.

When trans fluid gets uber cold, its considerably thicker so there is more "balloon" pressure inside the converter and would cause a weak vane to come loose. Best bet is to pull the trans and swap out the converter before it shells out completely and pumps metal particulate all through your trans rendering it a smoldering pile of oily wreckage .
 
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