um...chevyaudio, are you sure you're not using a turntable?
low bitrate MP3's dont raise the noise floor, in fact that would result in more data and a larger file size, the exact opposite of what the low bitrate MP3 is for in the first place.
Most likely its poor MP3 decoding hardware in your equipment, or just the amplifier.
No, the way to notice low bit rate MP3s is in distortion. Easiest way is in cymbal crashes, which sound very one-note-ish and electronic. Unfortunately once you start hearing this you'll be forced to re-rip your music collection and use a higher compression. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif