Crap, this really sucks. The noise is driving me nuts (mind you, its starting to get colder here so my windows are up more now then they were a month ago). Mine too can only be heard once you start driving (cannot be heard in neutral or in gear with brake on) in low RPM's when cold.
I was thinking I might try to take them off with the help of my Father who is a welder by trade (and does teh najority of his own mechanic work) but it soulds like it might be too big a chore.. and would hate to break or strip a manifold bolt.
Man... I hate this noise... and the cost!