Why Does My Transmitter Hum?

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 those in the know, these causes might include:

• Power supply hum due to poor DC supply filtering
• AC hum pickup in the transmitter audio due to poor shielding
• Ground loops
• RF induced hum modulation on the transmitted signal

What's the cure? Here are several links to technical articles dealing with hum reduction in audio and RF circuits:

Finesse Voltage Regulator Noise! from Wentzel associates
Hum reduction info from northcountryradio.com
Hum redux, from www.kenneke.com
Sound Card Interfacing from www.arrl.org
Hum Rejection from www.bluejeanscable.com
"Tweaks" - bottom of the article
Ground loops from www.epanorama.net
Breaking ground loops from www.epanorama.net
Hum Update from lists.contesting.com
Circuit tips - middle of the article
Power and grounding techniques
RF/RFI & Grounding

Happy Hum Hunting!