check EVERYTHING- if possible find a pump in a quite road so you can look down when you hit the bump. it sounds like it might just be a brake cable housing slapping against the bar or stem? I had a creek like this that ended up being a losse front wheel quick release. I had another the other day- a humm on rough roads that ended up being a loose bottle cage bolt. I used to store a spare spoke inside handlebars til that rattle drove me just nuts. I guess my point is it can frwquently be the stuff you've never considered, so try virtually everything.