Thanks, Witt! Actually I only heard it while taxiing, when the engines were at relatively low power settings. Plus, the device making the noise was pretty close at hand on the flight deck someplace, evidently. Or right below the floor boards. During taxi I didn't have my hearing protection headset on because it wasn't that loud, and this pump noise was quite distinct. While in flight, with the engines at flight power settings, and my hearing protection thingies on, I never did hear it. But maybe it only operates on the ground. It was pretty loud...
I found this in my B-17 pilot’s manual. The tank shown looks just the one the flight engineer pointed to when I asked about the noise.