8Khz vs 48Khz audio sampling and noise expectations
It sounds like there are two things going on here.

At the lower sample rate, high frequency noise that was previously picked up as such and subsequently filtered out now aliases into frequencies you can hear.  If you want 8 kHz sample rate, sample at a higher rate, then low pass filter before dessimation.  You want the filters to attenuate frequencies beyond 4 kHz, since that's the maximum a 8 kHz sample stream can reproduce anyway.  Anything above 4 kHz when you sample at 8 kHz is going to become noise that you can't really get rid of later.

The metallic sound is usually a consequence of low resolution, like 8 bits.  The quantization noise at 8 bits does sound sortof "metallic".  Try sampling with more bits, like 12 or 16 per sample.