I have a Taranis QX7 Tx.
Failsafe is set to "Custom".
The Rx on my plane is a Frsky RX8R.
When I turn off the power of my Tx, I get this strange noise/behavior on my Rx.
If the failsafe is set to "Receiver" on the Tx, everything is all right.
See the video here: 

Thanks.