Because it doesn’t know the words! Or, it could be that you are suffering from one of several technical causes of hum in transmitter circuits…
According to the folks at North Country Radio, there are several causes to consider. These causes include:
What’s the cure? Here are several links to technical articles dealing with hum reduction in audio and RF circuits:
Hum reduction info from www.northcountryradio.com
Pixie II tips, including hum redux, from www.kenneke.com
Hum Rejection in Unbalanced Audio Cables from www.bluejeanscable.com
Ground loops from www.epanorama.net
Breaking ground loops from www.epanorama.net
Hum Update Listserve for Hams from lists.contesting.com
Happy Hum Hunting!