Hi,
I have built a Arduino clone that uses a switching power supply because the input voltage might be as high as 20V and also to learn

. It kind of works but when I power the board, I hear a very annoying high pitch noise.
The power supply is based around the MC34064 that Dave described in a video a while ago. It's being used in step-down configuration to get 3.3V. I have a Vin of 18V (if I lower Vin to 8/10V, the noise is significantly reduced but I can still hear it).
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The values I have used are:
L: 33uH (should have been 36 but I could not find one)
R1: 2k
R2: 3.2k
Ct: 85pF
Rsc : 0.240 Ohm
Co: 10uF (I did not use an electrolytic here but a ceramic, could that be a problem?)
Diode is a B220A.
Where could this noise come from: the self? the diode? a capacitor? What can I do to diagnose that and reduce that noise?
I have attached two captures from the board. In red is the voltage at the output of the MC34063, getting into the self and the diode. In yellow is the "regulated" voltage measured on the board.
You can see that the output voltage goes from 3.04V to 4.24V that's a 1.2V "ripple" ... And also the switching frequency seems to be around 5kHz but is not very stable.
I should also say that there is not much power consumption from the board. Just an Atmega 34U4, a few pull-ups and a some leds right now.
Thanks for your suggestions!
thomas