Hi,
with a 50cm wire place as antenna in bnc and a transceiver near i can't hear the fm +/- 5khz modulated signal from 1 khz tone.
If i setting +/- 75 khz deviation and hear with a standard home fm receiver i've a cristal clear note on speacker.
So i suppose a issue in low level modulation.
if you have a HF transceiver make yourself this test..
Note :
for 88-108 Mhz setting are Fout 10,7 Mhz (i.f.) deviation +/- 75 khz tone modulation 1khz (heard Ok)
for transceiver Fout 24 Mhz deviation +/- 5khz tone modulation 1khz (no audio in transceiver)