What is causing my car to make a metal grinding noise?

It is a 97 mitsubishi eclipse spyder with 145k miles. It runs just fine but the only problem is a metal grinding noise. It only happens in 3rd, 4th, or 5th gear when i let off the gas. As soon as I push the gas back in, or throw it into neutral, it stops. It also make the same noise when rolling in neutral, it is more of a metal thumping noise that seems to go with the tire rotation. I took it to a local shop and they said they couldn't detect where the noise was from but would replace my transmission for $2300. I'm lost on this one! I think it's a bad wheel bearing or maybe a cv joint. I'm thinking the reason it stops when I put on the gas is because it has something to do with the compression. Any ideas?