Bang-bang music
From The Big Bang Theory Wiki
In The Hamburger Postulate:
After the String quartet of the Physics department had their practice session, Leonard and Leslie spent a night together. The next morning Howard told Leonard:
Howard: The blogospheres are buzzin' with news of you and Leslie Winkle making „Eine kleine bang-bang Musik“.
"Eine kleine bang-bang Musik" is German and translates to "a little bang-bang music".
Most likely Howard is referring to the famous Serenade No. 13 by composer Wolfgang Amadeus Mozart that is called "Eine kleine Nachtmusik", which literally translates to "a little night music". So Howard is making fun of the combination of classical music and the hook-up.
