i used to work with huge paper scanners, the main body of which was made from 2 steel I beams welded together to hake a tunnel. 1 end was sealed, what i did to get a nice metallic sound was put my digital mic in the structure and bang a spanner off the side of it. i gave a kinda 'doppler' sound. almost like a frozen lake cracking when stood on. when you put that in to the PC and filter it etc it sounded magical.