[TenTec] Argonaut 509 sidetone "pop"

[g p <n2wjw@yahoo.com>]

My 509 makes a "pop" everytime I key either a dot or a dash and it happens only 
with an electronic keyer and not with a key or a bug and the pop happens with 
any electronic keyer I have tried.
 
Is there a mod to fix this? Any help appreciated.
 
Gil