I have been looking through few car websites today and Avast Internet Security was telling me that these sites contained insecure content but I've been using them for a long time:
<snip>
Anyone experiencing similar issue? It's really weird and annoying.
This isn't something that I believe avast does, it alerts to infection, not insecure content, whatever insecure content is. Generally I would think that insecure content is 'mixed content' if you are visiting an https page and some content on that page is not from a secure 'https' location.
So as has been suggested a screenshot would help.