Knock after cold start

My 1999 Navigator has always had a brief "rod rap" on intitial start, but at about 106,000 miles the knock is louder and more prolonged (up to about 30 seconds) than ever. I discussed the knock with the dealer service dept. when the vehicle was new and was told it was characteristic of the engine.
If anyone has a similar issue, I'd like to hear from you. I'm assuming the knock is related to excessive rod bearing clearance prior to oil delivery to the rod; the noise might be from another source such as a cam drive chain, valve tappet or rocker arm......although my hunch is that the knock occurs at crankshaft speed and not cam speed. Oil changes have been at 2,500 miles and always a quality 5w-30 weight oil.

I had a new 1997 ford 150 with a 5.4L and it did this from the first oil change at 1000 miles I went to a stiffer oil. Try 10W30 oil this should help. My problem was the oil pump wouldn't prime on 5W30. I would be running 800rpm with no oil pressure for about 30 seconds.

Thanks for the tip!