I think it's the one closest to the transmission. When I had it taken in for a recall, I asked them to listen to the noise, and he said it's a bad mount. Didn't say which one, but The one that's on the driver's side that is visible looks fine, so i'm assuming it's a different one.
I was thinking about it, you could also have a loose starter or something like that. I had a guy come in the parts store saying that the starter we sold him made the engine "jump" when he started it, and the starter gear was going too far out, and hitting the flexplate when it was starting.