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I would double check with a different signal source as some TVs and some consumer RF modulators don't get along well.
If that doesn't solve it I would determine the exact sound IF frequency and set up a signal generator to create it. If it's SS lab grade it should have FM modulation capability to make audible checks. If it's a tube signal generator get a digitally tuned SW reciever with continuous coverage like a Sony ICF-SW7600GR set the radio to the exact frequency and tune the generator for best signal with worst signal coupling...I've found a lot of tube signal generators have AM modulation so crude it both produces FM components, and confuses frequency counters, but a good radio is hard to trick.
Once you have a signal generator set up signal injection troubleshooting/alignment becomes viable.
Might be possible to trace with a scope too, but signal injection is often easier.
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