1999 Siverado 1500 Makeing Noise

[BAD2500HD]

My son is working on a 99 1500 silverado and is haveing trouble with a noise he can't figure out , the truck was brought in to have the flexplate replaced which is what the customer said was wrong with his truck the work was done and it is still makeing a sound like the starter is still engaged or a grindding type noise which is what it was doing before , they replaced the starter ( which was found to be bad ) and a u-joint in the rear but no help still a sound . any clues as to what he should look for or check out ? he was told something about the torqe converter not being on right but said he has done this before and knows that it has to go on right which he said he is sure it is so any help would be great , thank you