midi sound issues in GarageBand

Hey

I just got a new MacBook and a midi keyboard - recorded some simple tracks and noticed an issue with the audio. When I played back something I recorded, there was an underlying sound that is reminiscent of a horse trotting (best way I could describe it).

It was an intermittent issue - I played the tune in Garageband later and it sounded perfect. I exported the tune as an mp3 and started playing it in 'Music' - and the galloping noise is back (again, intermittent - depends on when I play it). I thought this may be some kind of issue with my midi keyboard. So I unplugged everything except power (no keyboard connected), and started a new Garageband project. With just one track (the default Classic Electric piano) I opened musical typing - and when I hold a key down, I get the galloping noise. A few minutes later its gone for a short while before coming back.

I've done a diagnostic on the machine and it says all is fine (its a brand new Mac). Also tried reinstalling Garageband and also tried it in Logic where I experience the same issue. But then, I also get the issue just when playing the mp3 outside of GB! No other issues with playing audio on the machine eg from Spotify or YouTube or anything.

Any help much appreciated