I recently bought a new Classic Vibe Mustang guitar, and noticed that I could often hear a hum through the amp whenever I wasn't touching the strings or bridge.
The first thing I do after buying a Squier or other inexpensive bass/guitar is replace the born-to-fail jack socket with a decent one so I opened it up and soldered in a Pure Tone socket. While I was in there I also protected the control cavity from slug infestation by lining it with 25mm self-adhesive copper anti-slug tape from amazon. After doing these two jobs, the hum was now totally gone, so I deduced from this that the previously heard noise was probably from slugs in the control cavity humming along with the music.