[ICOM] Need SB-303/401 Expert Help

[OTAKEBI at aol.com]

I have been working an SB-303 SB-401 combo.
What I thought was mic hum is not.
I hooked up another receiver to hear my own signal and I can hear what  seems 
to be a tone
that I can only describe as a spotting tone type of noise when I dead  key 
the mic.
The noise frequency changes as I tune the transmitter with the tuning  knob.
If I leave the SB-303 on and unlock the transmitter (SB-401) to make it a  
stand alone transmitter the noise is gone when I dead key and the SSB  signal is 
clean on the receiver I am using to monitor this problem when I  talk.
This tells me it is not the transmitter, or is it?
Could an expert on this weird problem lead me to solving it.
Any help would be great since I am stuck and need new ideas.
Thanks,
Dan/N4VET