removing a jack post... proper support?

We installed hardwood floor 3 weeks ago. The sub-floor is OSB. It is quite level, so we didn’t put a layer of plywood on the OSB. We put wax paper and lay hardwood on top of wax paper. We have not used the room much since the floor was finished, because the baseboard was still outstanding. In the evening (after work), we walk around the room and the HW floor has light cracking noise initially. Then the sound disappear after 1-2 minutes of walking on the floor. What is that sound? Can it be an indication of installation problem or is it normal for the floor to settle? Note: The room is right above the basement. The basement temperature is certainly a bit lower. In the morning, the sun goes into the room and may warm-up the HW floor.

This house was added onto in the living room about 40 years ago where a side porch with cement floor was. ...well floor still there just part of the living room now. I have just removed the carpet in this area and see that the transition from wood to cement floor looks like they just leveled out/filled seam with morter but now after all these years in places the mortar has cracked and busted and there are some gaps about 1 inch wide and worst appears to be about 3/8 inch deep (haven't picked at it yet to makes sure all loose pieces are out). Most is just a small crack running along wood and cement. Whats best way to (well lets say budget fix and may last 15-20 years) fill these back in? I figure just remove loose pieces fill in with mortar or cement and maybe touch up with leveling compound to even/smooth it out. Would some-type of a cement/mortar crack repair be better? I'm going to install the vinyl planks (resilent) when/ if I ever get all the old padding removed from the wood ...thanks!