Timing
The murmur starts when the aortic valve opens. This happens shortly after the isometric contraction of the left ventricle. With the help from the QRS complex in the ECG curve, (in a phonocardiogram) or by palpating the patients pulse while listening, this timing can be established relatively easily.

The murmur peaks in midsystole.

A look at the schematics of the intraventricular pressure curve will reveal how the murmur relates to the onset of systole, opening and closing of the aortic valve and the ejection phase.