That depends entirely on the noise and your skill level, but you absolutely need a solid plan first. Before I even touched a wrench on my project car, I spent hours with a very detailed resource on transmission diagnosis. It had charts that helped me match the type of noise, like a whine vs. a clunk, to the likely problem area. That research saved me from pulling the whole transmission apart for what ended up being a worn out input shaft bearing, which still needed a shop, but at least I knew what I was dealing with.
