Structural boards in attic

I want to add on to ben's answer. You will always have sag with a 14 foot header. If it is a metal i-beam maybe 1/100th of an inch... Your wood/LVL will sag much more.

What you are describing is ultra typical when there is a point load above and they used LVL. I don't know why cities keep allowing the use of LVL for anything more than 10 feet. It will look great and then year 2-3 it starts sagging... year 4-5 you get cracks and notice it... It is not unsafe because the LVLs will have to be seriously sagging before anything would happen. 5/8" is just enough to disturb the finish and annoy you. You can either install a beam in the middle of the room or put in a better header.

I can tell you I bought quite a few houses with sagging rooms for a huge discount. It is really an easy fix. You support each side of the room with a temp frame job, cut out LVL, and install metal i-beam. This is a one day job if you have a couple helpers. The main thing is to jack the metal beam slightly higher than level and support that level during install.

They could simply be thermal expansion and contraction, delayed due to the large heat capacity of a home. It takes quite a while for the solar gain that has soaked into an attic to be released, for example.

I'd stake out your attic Columbo-style and give it a good listen and feel. Chances are it's not a sign of structural problems, but it's surely an annoyance.