Hi FCB,
Since you can't share your circuit, I will share mine:

If I run this model I get:

Which includes the start up behaviour.
If use the FFT function, the default is:

and the resulting FFT is:

If I look at the FFT for the last 10ms, that is from 10ms to 20ms I get>

Which doesn't include the start-up.
What do you want to do? using my model an example.
Regards,
Jay_Diddy_B
fft transient.asc (0.62 kB - downloaded 53 times.)