My tranny makes a funny grating sound in OD at low speeds (40 mph), and also sometimes in reverse. Other than that, it seems to shift fine. What could cause this?

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1990 IROC 350
Mods: Too busy trying to make it run right to mod it.
Airfoil, Dynomax cat-back, MSD coil, 180 t-stat, Bald Eagle tires,
Hypertech fan switch, Accel 23# injectors, Holley AFPR, ported plenum,
Ported Daytona Yellow stock base, Moroso valve covers, other stuff,
Ruger P95DC, hot wife, new oil filter, thick rubber floormats, no cats.
18.0 @ 85MPH since I'm one big-a$$ MF
"It's better to have and not need than to need and not have."

 

I think its just the noise your car makes when its warming up. But also be careful, my tranny made noise like that just before the pump went.