[Heathkit] SB-303 question

[OTAKEBI at aol.com]

Is it normal for an SB-303 to produce hum when the AF Gain is turned all  the 
way down.
The hum goes away when the AF Gain hits about 10:00 o'clock.
A fellow told me that it was about normal for that rig.
I can't believe that.
I have set it up outside (cables to and from  SB-401) and inside (on the 
board voltages).
Oh, and yes the hum is present with the SB-303 not connected to the SB-401  
and with a 50 ohm load on the LMO.
Any suggestions?
Or, is it the nature of the beast?
Thanks,
Dan/N4VET