Another alternative is to apply some light oil such as 3 in 1 or even a squirt of WD40 to every pipe union. If air is being sucked in at any point, the oil will be sucked in as well and temporarily stop the whistle..
Good tip. Failing that, I'll return to the old gasket after its had a good clean and see if that cures it. I read on a Honda forum that someone developed a whistle when they upgraded their TB and used a thermal gasket as well. It may be that there's a slight over hang into the chamber as is causing a resonance when it's opened.
