The "surprise" in haydn's symphony no. 94 occurred in the movement.

Haydn's Symphony No. 94 (The Surprise Symphony) was one of Haydn's twelve London symphonies. There are numerous musical jokes and surprises sprinkled throughout the work but the most famous surprise appears in the second movement. The second movement opens with a quiet violin section, with eight bars played piano (quiet) followed by eight more bars played pianissimo (very quiet). Suddenly, at the end of the 16 bars, the rest of the orchestra joins in for one single G-major chord played fortissimo (very loud). This surprise only occurs once in the piece and is not repeated.

The answer here is hills.