I did the defrag Mac, but that wasn't it.

I think the problem is due to RD tracks on max volume (127). I've been rendering with "Adjust Audio Volume by" set to 30 dB - the maximum allowable.

If I either reduce that setting to 27 or so, or reduce the RD volume to below 127, that seems to get rid of the repeated 'scratch' in the render

I'm not sure why the 127 and 30 settings should lead to that scratch sound tho ?

Thanks again - Best - Andy