I completely disassembled my Kayfun Nano, found the air intake in the 510 pin was slightly off, repositioned, & opened up the side air inlet a bit. Replaced o-rings - no more whistle. Both of my Promotey v2 got total overhauls, too. I found the negative post on 1 of them a bit loose, which could've caused a bit of a whistle. I reamed the air holes in the center posts, both in the tanks & bases, enlarged, then tapered & smoothed the side air inlets. Replaced all O-rings. No more whistling in these either. I still have a couple of tanks left to do, but, thanks to your recommendations, I'm not stressing about them - I know I can fix them with just a little time & effort. Thank you all for your suggestions - they worked!