Square wave function in FFT spectrum
Looks like aliasing. 

Your 360 Hz is probably not a real component at 360 Hz but it's the 9th harmonic (540 Hz) that aliases back to 360 Hz at a 900Hz sample rate. 

That's easy to test: change the sample rate to, 940 Hz and see of the 360Hz moves to 400Hz or stays put. It moves to 400Hz, it's clearly aliasing.