The buzz when you switch on is not a problem, its supposed to spark for approx 1 second after switch on to clean the plug, but if you can hear it that loudly then its is shorting out somewhere outside the plug cap, I suggest you take the cap off and the plug out, fit them back together and lay the plug on the cylinder head, turn on the ignition and watch the plug to see if it sparks in the right place, if it does then turn the engine over and again check that it is sparking in the right place, I had an ignition go recently that would one second spark but not fire when the engine was turned over, I had strained the hall sensor wires and pulled them sufficiently hard that they where no longer making contact inside the ignition box, my fault entirely, new ignition abtained from Pe and all is well again.

Check that you can see the connector inside the plug cap, you may have squashed it flat when putting the cap back on at some time.

Mike