Which of these is a vibrating effect produced on certain musical instruments?

In music, tremolo or tremolando is a trembling effect. There are two types of tremolo.

The first is a rapid reiteration of a single note, particularly used on bowed string instruments, by rapidly moving the bow back and forth; plucked strings such as on a harp, where it is called bisbigliando or "whispering"; and tremolo picking, in which a single note is repeated extremely rapidly with a plectrum (or "pick") on traditionally plucked string instruments such as guitar, mandolin, etc.

The second type of tremolo is a variation in amplitude as produced on organs by tremulants. Also using electronic effects in guitar amplifiers and effects pedals which rapidly turn the volume of a signal up and down, creating a "shuddering" effect.