I have a Powerbook G4 1.67GHz 1Gb RAM 15"

After having my machine checked out for sound output whilst it was still within warranty, I have recently discovered that the problem I had is still there: playback of audio files seems to be ok, but whenever I try generating sound from a synth or, in the latest case, simply generating a test-tone, I get this annoying stutter of intermittent sound, as if it is being passed through a tremolo, when it should be a smooth sound (especially the test tone).

When I use an external audio interface this doesn't happen, so it seems it is sth to do with the internal sound processor. If so, this should have been picked up when I had it checked over.

There is also an annoying 'clicking' sound that seems to be pretty irregular and happens on both the internal and external sound engines. Any idea what this might be due to?

Thanks for any help in advance.