i test drove a 99 SD w/ 7.3l before i found my 96 for less $$$,

it had a very loud noise @ idle. went away as soon as you hit the pedal. it was the original owner w/ like 70-80K. he said it was there since new and was under warranty so he drove it. dealer said it would not hurt anything.
it bothered me so i did not buy it.
but later on i asked my father ..a diesel tech and he said he had one do the same thing, it had been to numerous shops w/ out any luck.
he found it!!! it was some welding slag inside the downpipe. he pulled the pipe out and ran the truck...no noise,ground out the pieces and put it back on. truck fixed. do you have the stock exhaust???
it is an odd condition but it was seen on one other that i know of.