Bandpass filtering an audio signal with MATLAB's $\tt kaiserord$: unknown new frequency component
Your filter is a stop band attenuation of about 40 dB. Frequencies outside your passband will be attenuated by at least 40 dB but they will not go down all the way to zero. 

The FIR filter is very long (2000 or samples). It takes that many samples for the filter to reach "steady state". In other words, you should throw away the first 2000 samples of your filtered signal before doing any analysis on it.