Thanks for the input, Spyderman9803; sounds like you've confirmed what SpyderGS said.
SpyderGS: thanks for coming to the rescue again. I didn't post for awhile because my Expo was the in the shop of the 2nd repairman for awhile. I just got it back to drive. I took it on a 90 mile trip via interstate, and the loud hum was almost unbearable. Definitely couldn't have a conversation with anyone. As long as I kept the speed at 65, it ran fairly quietly, but when I set the cruise control on 70, it sounded like a plane taking off and never quieted down.
Now I have to decide if it's worth putting more money into it. Since I've put close to $1000 in it in the last month, I guess I'll go ahead and talk to the 2nd mechanic about working on the transmittion. Even though it won't be cheap, it's still better than having to get another car, and I really like this one. He did say it should get at least 300,000 miles, as it's been kept in great shape for the past 10 years (It has 169,000 miles so far). Thanks again for the advice, and I'll keep you guys posted. Wish you lived close by; I'd bake you some homemade bread to say "thanks!"
Joy