If your w/l comes on before you start and goes out after starting then the alternator will remain charging throught its rev range,fist thing that i would do if it were mine would change the belt unless it was a new one fitted during layup. 2nd thing would be to tighten the belt as I suspect that at that critical speed its whipping ,increase the revs and the whipping stops and all appears to be ok. 2nd option is the cheapest so try that first.