I use a whip antenna on my service monitor output to supply the weak signal. Adjust the output till you have about 10 dB of quieting with the squelch open. Any additional hiss is that is heard when the transmitter is on the air is white noise from the transmitter. If you feel the noise level is excessive touch up the notches. Most repeaters I have heard do have some audible white noise on the receiver.
Perfection is hard to achieve in the real world.
Worry about it only if the noise level is so high that it obscures the weakest modulated signal that you could normally expect to communicate with. For example, lets say that you can clearly hear a .25 uV signal on your receiver with the transmitter off. With the transmitter on the noise level increases 2 dB. This you could accept after final tuning. If the same signal disappears completely when the transmitter is keyed, then the desense is excessive and there are still problems in the system. Copyright 1999 Remec Wacom.