You can also simply have a dirty contact in your 2 position e-unit - it will still make noise as you stated, but it can run badly or not at all.
A very simple thing to try is to wet a Q-Tip with alcohol, then swab both e-unit contacts - the e-unit contacts are simply the back side of the solder stakes on the e-unit. It takes less than a minute and IMO it's worth trying.
George