Wow -- 140 million miles :)

Wouldn't worry about it. Several, including mine, that do that.

I just dealt with apparently the same rattle on my '97 Altima 2.4L (135K
miles). The dealer and an independent confirmed that it was timing chain
noise which would cost a minimum of $750 to fix because of all the labor
involved. Nissan's Technical Service Bulletin NTB98-055a dated 15 November
1998 tells the story. The tension side chain guide needs to be replaced on
the lower timing chain, and the guides removed from the upper timing
chain. I decided to roll the dice and see if the noise gets worse rather
than fix it. Apparently the lower chain guide is partially plastic, and
can damage the engine when it self destructs. Good luck and let's keep our
fingers crossed!
PS You might want to consider switching to synthetic oil (like Mobil one)
if your not already using it.