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.
The Big Bang Theory episode where Sheldon Cooper plays the theremin is entitled The Bus Pants Utilization.
Sheldon gets drunk in Season 3, episode 18 named "The Pants Alternative".
The Einstein Approximation Season 3 Episode 14
episode 8, season 3, 'The Adhesive Duck Deficiency'
In the episode were she visits him in Pasedena, her name is Missy.
The Big Bang Theory episode where Sheldon Cooper plays the theremin is entitled The Bus Pants Utilization.
Sheldon gets drunk in Season 3, episode 18 named "The Pants Alternative".
It was an actor
In the World of Warcraft episode of "The Big Bang Theory", Sheldon's name was Sheldor.
The Einstein Approximation Season 3 Episode 14
episode 8, season 3, 'The Adhesive Duck Deficiency'
In the episode were she visits him in Pasedena, her name is Missy.
That episode title is The Bath Item Gift Hypothesis.
Season 3, Episode 22: The Staircase Implementation
In Season 3, episode 16 on the episode entitled The Excelsior Aquisition, Sheldon goes to jail for contempt of court after insulting the judge while fighting a traffic ticket.
In the episode titled "The Einstein Approximation," Leonard states that Sheldon has been emotionally stuck for "about 29 years." The episode aired in 2010 February, which would mean Sheldon was born about 1981. Jim Parsons, the actor who portrays Sheldon, was born in 1973.
Uh.... I don't understand your question. I don't remember an episode of "The Big Bang Theory" in which Sheldon and Leonard did that to each other.