What on earth do you mean by a rhythmic sound? You need to be a bit more clear as we are not there and can't see or hear what you can. The only thing I can suggest is tip it back far as possible or lay it down, so you can access underneath. Take hold of the centre pulley and see if it turns backward and forward. If it is not free to turn either way then you have a clutch problem.i.e. something has broken in the clutch or the clutch spring has seized.
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