The problem is your ballcock (fill valve). When the fill valve diaphram gets old, it begins to loose its ability to shut off completely. So it shuts off then turns back on and so on at many beats per second. The result is a harmonic vibration in your piping that sounds like a tuba. I would suggest one of two things. First you can replace the diaphram in the fill valve, but it has to be done right. Second you can replace the whole ball cock for about 9 bucks. If you have good water pressure, use a Fluidmaster 400A. If not, use a Masfield float style ball cock.
They are easy to install. Always buy a flexible braided supply tube at the same time and install it along with the ball cock. And do not forget to replace the flapper at the same time. You will never get a better opportunity.
For help with the install, look at www.toiletology.com.