[TenTec] Paragon II RS-232 Port

[KA6UUP]

I tried hooking the Paragon up to a computer / Ham Radio Delux via the 
RS-232 port the other day and was successful.
However, there was a very noticeable "scratching " noise present in the 
speaker audio.
Using the computer keyboard had an effect on the noise and so I assume 
this is computer interference getting into the radio
through the RS-232 circuit.
Is this typical?
Is there a fix for this or must I just get used to it?
Tnx es 73,
Chuck KA6UUP