Regular squeak from rear wheel My back wheel is giving out a periodic squeak as if the wheel were buckled and rubbing on the brake.  Only, it's not buckled and it's not rubbing on the brake.

It's one squeak per revolution of the wheel, regardless of speed, but each squeak is the same (short) duration even if I move slowly.  It doesn't matter whether I pedal or coast.  The wheel isn't rubbing on the frame and if I gently brake through several revolutions it still squeaks while I'm braking.

I asked a bike shop about it a while back and they couldn't find anything but after all the routine adjustments it went away for a while.  Now it's back.

If I get off and turn the wheel by hand there's no sound.  Even if I lean on it and roll it around it's quiet.  It seems that I need to put all my weight (an my panniers and gym gear) on it to make it squeak.  I guess that's why they couldn't find it in the shop.

My best guess is uneven spoke tension might allow some of the spokes to shift and rub slightly.  I've looked them over and don't see any wear and I'm not sure how I'd make a definitive diagnosis there.

What could it be?  What else can I test?
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.