Why does my car rattle when starting

What is your question?

Almost every time I start my car it rattles even during driving I was told water in the tank and treat it when I fill up with gas well I've used almost atleast 7 bottles no changes low octane what could this be

What makes this problem better or worse?

When starting sitting sometimes not all the time popcorn sound almost

How long has your 2013 Chevrolet Impala had this problem?

Couple months