I've been rebuilding the front wheel from a 1989 Fuji road bike. At the moment, the wheel runs about as true as I can possibly get it, though it's not at full tension yet. However, when I pluck the spokes on either side of the wheel, I get a lot of variation in pitch. I'm lucky enough to have near-perfect relative pitch, so I don't believe it's an error with my ear. Could it be that the rim is pretty old? I've heard of this causing problems, but there seems to be a rather large variation in pitch for something like this.