Engine Knock Right at Engine Start only!!

I'm trying to get as much research for this as I possibly can before taking
action. Any info you guys could provide would be greatly appreciated!

My Situation:

Sometimes when I go to start my truck, I will turn the engine over and
when
it fires up there are a series of 4 knocks and then it goes away. Each and
every time it does it, it's four knocks. It usually happens after the
engine
has been sitting for the night. I took my truck to a mechanic concerning
this
"knock". He said it sounded like front/rear main bearings or possibly the
crank shaft, but he could not tell which area it came from. What puzzled
him
is that it only "knocked" four times and then went away. After that,
everything sounded perfect. Absolutely NO ENGINE NOISE WHAT SO EVER. He
commented that the truck ran remarkably well.

Info I recieved from another forum:

this is most likely a problem with the crankshaft main bearings. general
motors had a bulletin on this when these engines were new and the fix was
to
measure the main bearings and install oversized bearings on the mains.
they
also note that this will not affect the longevity of the engine. bottom
line
is leave it alone.

Do you guys concur with this response? 

Thank you for your input!

Phil