The Knitting Sequence are

1. Needle holding an old stitch loop at rest position
2. Needle casting off an old stitch loop at clearing position
3. Needle forming a new stitch loop with a newly fed yarn at knock over position
4. And repeat.

The first occurrence of the multiple repeated knocks appears in Series 1 Episode 10 - The Loobenfeld Decay:

Sheldon: (Knock, knock, knock, knock) Leonard, (Knock, knock, knock, knock) Leonard, (Knock, knock, knock, knock) Leonard...

Sheldon: (Knock, knock, knock, knock) Penny, (Knock, knock, knock, knock) Penny, (Knock, knock, knock, knock)Penny, (door opens) Good morning.