can anyone help? (down pipe)

can anyone help?i recently had a down pipe fitted to my 1990 cab to eliminate an annoying noise, noise remained, and was advised the exhaust system may be knackered. replaced the system with a jetex from the down pipe, no difference to the annoying sound.
the fellas at demontweeks tunning reckon the down pipe is noisy because it is not original so where may i find an original one???? at the end of my rope with it now!! the noise seems to come from the bend before the joint in to the jetex.
thanks for any help or advice.