I have a 40 kHz ultrasonic transducer that transmits a pulse. The pulse is generated by switching a phase shift oscillator on and off with a NE555 timer. For some reason the sender is making a faint audible noise. This is the first time I am noticing it. I thought the sender might be damaged but the pulse at the receiver seems  fine although there is more coupling at the receiver than before.

Any idea what might be wrong? 
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  [1]: https://i.sstatic.net/xxlaj.png