Quote Originally Posted by sina518
i was taught by you.
Ditto, you both know exactly what you are talking about, the entire body of the vehicle is a ground conductor which can induce noise and is typically the problem.
http://www.carsound.com/columns/navone/nav0005.shtml

www.canze.org.nz/forum/attachments//Trace_Noise.doc
here is a link to help you in the process of elimination.