518 Screech in neutral

Yesterday while running around town, when I put my 518 in neutral it (or converter?) responded with a very loud screech. (I had a similar quite disturbing sound once in the past, but it turned out the flex plate bolts were rubbing on my dust cover, removed it and left it off.) I pulled off to a parking lot and it still did this anytime in neutral. I got out to check the fluid, then realized I didn't want to sit there with it screeching like that if I put it in neutral. I had about 1/4 qt. of fluid with me, added that to see if it helped. Never made the sound again! Came home to other things going on, later when fairly cooled down, I started it, put it in neutral and checked and it read right at the full mark. (I don't know what else would have made the "neutral specific" noise?)
Any ideas???
(340 with a Cope Racing Transmissions build, 11" converter from PTC 2600 stall.)