Why my amp creates a fizzy sound??

[jeffrey0603]

Recently my amp, line6 spider III 15W, has been acting funny. When i turn the master volume control, it becomes EXTREMELY fizzy. No its not the gain, i am sure of it. When i turn the control, sometimes its fizzy and sometimes its normal, i need to keep turning until the sound becomes normal. You guys know what i mean? Its kind of like the radio and you need to fine tune to one frequency to get a non-fizzy sound. I think it needs repairs.

Hope i made sense.