I did that once to an old Megane's cassette radio. You have to somehow deactivate the tape part for the noise to stop. At least that worked in the radio I was working on.
In my old Ph3 306 1.6 I modded the original CD radio in this way. Inject the audio signal from an RCA or 3.5 mm cable into the audio path that goes from the CD reader unit to the audio processor on the radio mainboard, burn an audio disc with 80 mins of silence, put it in the radio and enjoy. Don't forget to put some resistors in-line (L/R channel) to protect the IC depending on where you solder the wires to. I used something between 500 Ohm and 1,5 kOhm. Maybe I find some pics.
EDIT: Unfortunately no pics from inside, only from outside from when I put an ad up to sell it. I ran a cable with a female 3.5 mm connector out of the rear of the unit so I could connect it to a jack i mounted inside the tray above the ashtray. A second 3.5 mm jack I mounted to the faceplate so I could have music from my MP3 in the tray at the same time as e.g. notifications from satnav, mobile etc. mounted on the vents. A jack from an old Nokia fit perfectly.