Newb Question

[Jimmynewbie]

Greetings to all, from Canadas' east coast, have recently bought my first Jimmy ( 2003 SLS 4x4 4.3l Vortec ) , after driving for the past couple of weeks I have noticed a "flasher" noise under the dash,I have checked all bulbs etc. the sound still persists, it disappears when I apply brake pedal but reappears when foot is off brake....I could turn up the volume control on the radio , but I would prefer to determine the cause of problem and correct by DIY.... Any advice or suggestions greatly appreciated.... Thanks, Jimmynewbie