You may have a ground loop problem. I suggest the use of an isolation
transformer between the transmitter and receiver. Also pay attention
to signal levels, an attenuator be may be required to achive the
proper audio input level to the transmitter.

Jimmie

Thanks Jimmie. I'll check that and let you know. If I can, I'll run
some tests at my university where I can use the oscilloscope. I'll be
sure to get some images in case the transformer doesn't work.

Philippe