The hammer pivot needs some oil. My new SBL also squeaked. A squirt of oil into it cured it. But I always take down a new firearm all the way. The machine work inside is superb and clean. And while in there I oiled and greased as appropriate. Also, if a little oil does not stop it on the pivot, pull the stock and oil/grease the hammer spring and strut and retaining clip.
BTW, if my 1972 JM Marlin 336 Texan goes without cleaning, the hammer will let me know with a similar croaking, skronking sound. It just wants some TLC. So when my SBL had a squeak I knew right away what it was asking of me. I guess maybe Ruger was a little to sparring with lubrication. The squeak will go away as the rifle breaks in.
While you are at it, pull the lever (see your OM) and put a little Mobil 1 grease (or your favorite gun grease) on the snail cam and also on the finger plunger. It will help these parts to break in smoothly.
With a JM and especially some of the earlier Remington models some parts needed polishing. I would refrain from that on the Ruger-Marlins, I found nothing in my new SBL and my new GBL that needed anything other than a touch of lubrication. They are near perfect.
I use Ballistol and Mobil 1 red grease on most of my guns. If you prefer CLP or similar that will do as well or better.
Your OM:
Edit to add, see that hammer coil spring and the hammer strut that sticks through the retaining clip? Sometimes if the hammer spring is dry it will make a squeaking or skronking sound as it moves over the hammer strut or the strut will as it moves in and out of that slot in the clip. Yes, lightly oil or grease those things.
