Noises. They are the toughest nut to crack. Sometimes you just can't narrow down what it is the person is hearing. What is abnormal to them might seem perfectly fine to you. Noise diagnostic without the customer riding along with you, so they can point it out when it does happen make it even tougher.
Some rules I like to live by.
I want info, lots of it, I want to be a journalist. Who, What, Why, When, Where and How. The more the better.
TSB's help, looking them up is one of those first steps you should do.
Test drive with the customer. When they say it happens "all the time" then have them drive and show you. When they don't have it happen all the time then more information will come out.
There is more, but usually these three will help you get on the right track to find it after it is racked.