Most of them are noisy because they are faulty. When inductance is too low (internal short circuit between turns or layers) , there are high peaks currents who are stressing the ferrite core to saturation and the noise is coming from sub harmonics of magnetostriction of the core.

hmmm, thanks for that

So I can just measure the resistance (now I have the data from my old coil) of the windings to see if they are shorted or not...

How to measure an inductance without RLC meter when you have a function generator with counter, a capacitor of known value and a working oscilloscope ? This is very easy, I let you guess the answer......

A resonant circuit yeah why not... I am rusty on the theory/math though, I must admit... I will dig out my old text books... 1/2*PI*sqrt(LC), something like that... shame on me. I mean I studied this stuff 20 years ago, and zero practice since then... yes of course I got rusty !

No worries, I will get my old text books and refresh my memory

I don't have a counter BTW

My TDS 544A can measure frequency though, obviously...

I have a 1Mhz analog generator, and a 2MHz "DDS" Fluke arb gen but it's in pieces... I am in the process of replacing the original LCD, with a back lit unit, because the original LCD has no backlight and it's just a real pain to read...

Anyway, I could also just wire up on a bread board, a simple first order RL filter, look for the 45° phase shift / 1/sqrt(2) amplitude reduction (the TDS544A could help with its automatic measurements) and figure out what value L is...

Well OK, in practice it would not be as accurate as measuring the frequency of a resonant circuit, I admit...

OK OK....I will do a resonant circuit. Google refresh my memory please !

https://www.electrical4u.com/resonance-in-series-rlc-circuit/Yeah, phew, I was correct earlier, 20 years of inactivity have not erased everything just yet, I am not as hopeless as I thought...