It depent of the ferrit wat Voltage you get from the transformer.
I think I've try 20 or more ferrits with all the same size and the same distance of wire.
All ferrits are given a differend Voltage and some ferrit don't ocsillate at all.
There are HF-ferrirs, LF-ferrits and everthing between it.
Ferrits from powerwires don't do anything and ferrits from switchpowersupplys working great.
In the topic from JMJ you can read the tests I've make.

Same with e-cores, they don't work all.