Improve output quality of FM receiver TEA5767
Your mention of the audio still having noise when you are "scanning station (muted)", leads me to believe the problem could be your "cheap" audio amplifier.  

However, it is difficult to reconcile why you get acceptable audio when forced mono.  

Try putting a de-coupling capacitor (perhaps 0.1 uf) across the +5 volts of your audio amplifier (+5v to board ground). Put the capacitor right on the amplifier board if you can.