About some type glitch in the frequency Response

i sent a wave with three frequencies 1000,4000,6000...I passed it through the 1250 cutoff lowpass frequency .but i found a glitch at the frequnecy around 1300 Hz to 1400 Hz...why i am getting this glitch and what is this glitch ?

[Raj]

Information is insufficient to say anything.

1) Show your filter design.

2) Can you attach the full plot instead of a portion snapshot?

[Hari Ijjada]

carrier=single(Ac*cos(2*pi*Fc*t/Fs));

a=single(mod.*carrier);

%FPass=1200 Fstop=1500 Fs=25000 Fm=1000

N = 700; % Filter order

F = [0 0.096 0.12 1]; % Frequency band edges

A = [1 1 0 0]; % Desired amplitudessingle

b = single(firls(N,F,A));

filt_out=single(conv(b,a));

DCB= dsp.DCBlocker('Algorithm','FIR','Length', 100);

demod=step(DCB,filt_out');

This is my filter...mod contains three frequencies 1000,4000,6000 Hz...