buzzing sounds, record & playback

[Gringo Viejo]

I´m a newbie with a new laptop, just loaded v.6.9 on it. I successfully recorded a track which contains 4 minutes of a metronome clicking away. That`s how I began tracks last year because version 5.3 had no built in click. Then I tried to overdub a gtr + vocal track, but I get a very distorted sound in the headphones while recording and it`s worse on playback. It sounds like I`m playing back my track through a two inch speaker with a rip in the cone. My record volume is not too high, since the red peak light on the MBox is not blinking at all. Adjusting the mix knob on the MBox doesn`t fix anything, either during record or playback. I`m using the same mic I used to record the metronome. Thanks.