04 Sierra vibration under dash

I have a 2004 Sierra with 12,000 miles. There is a rattle comking from under the dash on the passenger side between 40 and 50 mph under slight acceleration. I have had it in the dealer service department once and the mechanic thought he had it fixed, but it has returned, am taking it back in tomorrow. I was wondering if anyone else has had this problem and if they have had any luck repairing it.
Thanks, Sawyer