Just had a look into datasheet of MC34063. They speak about 120mVpp ripple at 500mA. If his load current is much lower, then switcher possibly runs in discontinous mode, which can be the reason for the slight increase of ripple, he observed.

I wonder, why Damjan does not use this optional ripple filter, which is also recommended in datasheet??

Damjan should have a look at datasheets or application notes, which give detailed help for making proper layout! Otherwise he will suffer from high ground noise, where even proper ripple filter will not help.

Kai