How do I know what is causing the noise in my signal I have a noisy audio file, available here (It is safe to download if you wish to)
https://ufile.io/pstrt

Now, I inspect it in matlab:


And then i plot it on normalised frequency:


Now I need to filter this signal to remove the noise. However, I have no idea what is causing the noise in the first place. Perhaps it is obvious even from the two figures I have shown but unfortunately not for me. I could play a guessing game and apply different configurations of lowpass, highpass and bandpass filters, but I have quite a few of such audio files to analyse. Thank you for any help.
Since it's home work, I can't give the full solution. 


The noise seem to have been artificially added. This type of noise doesn't happen normally
The noise is basically two sine waves that really don't belong there
The best filter type to eliminate these are notch filters.