You may need to find a shop that has one of these:
Steelman Electronic 6 Channel Chassis Ear Listening Kit JS06600 - Arctic Tools - Discount Auto Tools

Used one in tech school years ago and wish the shops I worked in had one.
Someday I should by one for home use.

I had a similar noise in my 2000 2500HD. I felt pretty stupid when I finally tracked it down. It was the little plastic cover for the engine block heater cord peaking out from under the hood and flapping against the side of the grill just out of sight at irregular intervals (whenever the speed and wind direction were just right). I was shocked to realize how loud it could be.

[QUOTE=spankey;79125]The thing is, it doesn't get worse the faster I go, Its so random that we can't associated anything with the sound. It just happens, and happens randomly... and it seems like it has to warm up before it starts happening. for example, I can usually drive 5 minutes without hearing anything... then all of a sudden it will start going. Yesterday I drove for about an hour and it didnt make a peep, then out of nowhere it started to make the noise :-/

Mine does same with a heavy load hbu?