While most of the forum is using https, there are a few parts still using http.
Example:
http://67-72chevytrucks.com/vboard/i...dient_tcat.gif
Guessing this is a background color.
Also in the first red block a couple posts up, the script from apis.google.com is being called via http.
A bit further down, looks like an ad is trying to load from an http server.
Not the user's fault. The browsers are flagging any call to plain http as a possible security issue, even a rather safe call like the gradient gif.