Have noticed recently that when I first get in my car and turn on the a/c, once underway my engine has this "spark-knock" like sound for the first few minutes of driving and then it always disappears for the remainder of my drive. It does not do this at all with the a/c off at any time.

I am running as always 87 BP gasoline and have never had this problem before in any season of driving including last summer. .

Any ideas whats up?

 

Could be a bum load of gas. Go to a different station for your next fillup.