When you refer to it as a 'sewing machine' sound that is intermittent, that brings to mind alternator noise through the audio system amplifier. When you put your ear close to a speaker, does that seem to be the source of the sound?

If so, the typical cause is a bad ground, either at the ground connection of the audio system, or an engine-to-frame strap. The noise can often have a 'tapping' character to it as well.

Steve