Cold start rattle

My truck has done this for years. After starting, it will make a slight rattling noise for 1-2 seconds. It doesn't always do it immediately meaning that it sometimes starts 5-10 seconds after I start the truck. It's very random. Sometimes it won't do it for a month or more. Other times it might do it for 2 starts in a row. Time of year makes no difference. I've tried to record a video of it, but you can't pick up the sound from my phone. No metal shavings in oil and the last Blackstone sample was fine.

Thoughts on what this could be?

 

Engine is definitely going out. I'll take it off your hands for $500.

 

That is very generous of you.