Description is like the same sound as mind made.....did it for a while then stopped, thats when the rattling started......then there was nothing, not even boost....I could not describe it either at the time(I thought it was the gearbox)......I put the new turbo in, no more rattle and know more strange sound thats hard to explain. Tell me does you turbo seem as though it has multiple boost settings sometimes under full peddle?
P.S. Danny I got 40,000K's out of my turbo from the start of that noise....you will know when you are close as that will be when the rattling sound starts to really irritate you.