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
Editor's note: Dead links were recently updated with links to the original content found at the awesome, link saving Internet Archive!
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!