Potential foundation issue

[Soh Tanaka]

I found this ranch house built in 1950 with an extension to the side. It looks like the extension has been there for quite a few years. There's no issue looking at it from the outside. The problem is the place where the original house meets the extension, the floor is slightly but noticeably buckled up inside the house. Although it has a crawlspace, the entrance was too small to get in to see how the "seem" looks like from the bottom. I was wondering if this is a common issue that I can neglect, or I should worry about.

Thanks in advance!