Carbon nanotubes (CNTs) have many interesting properties for nano devices such as high sensitive sensors or noise enhanced nonlinear devices. A field effect transistor (FET) structure is one of the key features for these applications, and the control of noise in FETs is important for the actual operation of the application. Several origins of noise have been proposed, and defects and/or surface adsorption of molecules seem to be dominant for the 1/f type noise in CNTs. To study the origins of noise, the diameter dependence of noise properties was studied. We analyzed the noise properties in CNTs using noise spectroscopy with different fabrication parameters or ambient environments. We observed the crossover of noise properties in CNTs, which involved transition between different origins of noise depending on their diameter. Additionally, noise spectroscopy was used to observe such crossover between air and vacuum environments. We can control noise intensity using the gate voltage, and noise properties can be controlled by the fabrication parameters. These phenomena are useful for the stochastic operation of CNT-FETs. (C) 2012 Elsevier B. V. All rights reserved.