That does sound like synesthesia. I have sound -> touch synesthesia. There's some discussions about whether a pretty big percentage of people have some form or another actually
Not necessarily synesthesia. I think what OP describes is closer to Ideasthesia which indeed isn't as uncommon.
Welp... now i understand why I've only encountered like one other person that seemed to immediately understand what I was talking about when I used the word "thought-shapes".