Regular squeak from rear wheel
It could be the freewheel or hub bearing causing the squeak but this should not be the case as both are lubed,  or should be. More likely is as you mention a spoke. 
Sometimes spokes squeak at the hub end as they flex.. Try rubbing in some grease or putting on a drop of oil on each spoke fitting at the hub end to see if it cures it,  just ride the bike and let the lube take effect. 
Of course a spoke could also squeak at the nipple end too as it flexes slightly in the wheel,  the only way to address/lube this end properly is by removing the tyre etc and applying a tiny drop of oil to each nipple. Don't let it get near your braking surfaces though.