Here's a link to the post and a video on it wobbling.
https://photos.app.goo.gl/huCeNoR4D79aDTJR9
In one video, you can see the whole post is moving quite a bit.
As you can see in the photos, I have access to underneath the stairs. And it appears they put a piece of pressure treated lumber under that spot with the post. There is nothing visible in the area under the tread.
However, I don't know how this post is assembled or attached. There's no plugs or anything on the post itself that I could remove to access screw or bolts. I see some brad nail holes in a few places. The top trim piece doesn't appear to have any brad nails or holes in it. Should it just pop off with a little pressure underneath?
Best Answer
To remove the post, I had to remove the trim piece under the handrail. It was held in with very fine brad nails and I had to wedge a putty knife in there to get it loose. That allowed me to remove the handrail from the post.
Once the post was free, I was then able to slide the post out. It was fastened with the keylock type post fastener. The fastener was screwed into the tread and 2x4 underneath and the screws had come loose. I tightened them, but I suspect they'll just come loose again.
I think the long term fix would be to use long bolts with a plate underneath to keep them tight.