How reliable is the fuel on your side of the pond? What octane are you using and what is the elevation where you live? These can all lead to spark knock (which by the way, always occurs exactly when you say your noise occurs). You will know that your timing is set correctly if you hear a slight spark knock when accelerating under load that goes away once you reach a steady speed. You should try to retard your timing until the spark knock goes away, then advance it one degree at a time on your octane selector until a slight spark knock occurs under load. Timing that is set with too much advance will cause a backfire.
