So I've just recently purchased a 98 Firebird and while driving it home (6 hours away) it would make a grinding noise going in and out of lockup over 35MPH and under 70MPH. Found a few TSBs for it. Earliest one has you go back to the dealer and get it reprogrammed which sounds like a band aid and won't actually fix anything. The second says it's a valve body wear issue which seems to be the main issue. The third says the same thing, except it says to use a year later model valve body (Jan '00). I found out that the person who previously owned it (before the person I bought it from because the person I bought it from only had it for a few months and never got it titled because of emissions issues) had replaced parts in the valve body to fix this issue and it's still happening. At least it saved me some time and money because I didn't have to order the new valves and such. I ended up replacing the valve body with an '02 valve body and had to change out the force motor since they changed the connection type. It's much better, but still on occasion makes a grinding noise. I imagine it's part from the debris that's still in the transmission since even though I've changed the fluid and tried to drain out as much as I could, there's still a lot of sediment in there and probably still needs a flush. I also think a pump body change would be a good idea seeing how it has valve components that wear and can contribute to problems. I guess the best and fastest solution would be to just swap out the transmission for a used one.

Update from Feb 12, 2011: well it's back to making the grinding noise like it originally was. Seems like it may just be a lack of pressure from pump wear or something in the pump body so when I get the chance I am going to replace it and submit another update on the outcome.