at idle (~700 rpm) my exhaust makes a very loud noise like its shaking vigourously. I've had a couple friends tell me that my exhaust is loose somewhere and my other friend who had the exact same car (89 240sx) said that he had the same problem and got it welded to fix the problem. I dont even know where to start? any help?

muffler shop is where to start. if you need help with this i would consult someone who knows more than you.