If I had to guess, I would say it's coming from CVT somewhere. It's funny you mention that, As I did have it apart last weekend to see if drive boss was de-restricted.... well it was, so I put it back together. I've rode about 200 miles since then and all was fine until today...... If the nut came loose, wouldn't it do this at any acceleration then? Why just at 30+? and yeah, It sounds exactly like a weed whacker!