How to Adjust FFT Bins for Audio Equalizer in C Without Introducing Noise?

I am trying to build an equalizer on C, using fft (fast Fourier transformation), since now i am able to apply the fft correctly on the vector (audio samples) normalize it and then apply the inverse fft. since here all OK , but when i try to modify the result of the fft(to increase the gain of one or various frequency bands) i get just noise (even if i just slightly modify it).My problem is the next: im trying to build an equalizer on C, using fft (fast fourier transformation), since now im able to apply the fft correctly on the vector (audio samples) normalize it and then apply the inverse fft. since here all ok , but when i try to modify the result of the fft(to increase the gain of one or various frequency bands) i get just noise (even if i just slightly modify it). How can i modify this bins (real+imaginary) components, that mi fft is giving:(0,real, imaginary, real, imaginary...) to equalize properly?

James,
Have you tried decreasing the gain of one or more frequency bands?

Have you tried changing  (increasing/decreasing) all the frequency bands at the same time?
If so, what were the results?

I just did a Google search for "fft equalizer source code". I don't have time to look further but the results looked promising
If you could share a bit more of how you are going about this (flow chart or similar), perhaps we could be of more help.