Good thing I pulled the sump! The pickup was extended to match the deep sump (good thing) but the extension was loose on the cast in pickup tube (bad thing). Chance it was sucking some air! Hopefully this pic will make sense--the loose bolt sits proud of the extension. The pickup itself kept it from backing out all the way:
The extension was made of softish alu and the hex headed bolts had made impressions in their countersinks. I'll be spotfacing those countersinks, putting a proper washer in, and Loctiting the bejeezus out of it on reassembly.