m40x's/m702's are known to occasionally whistle due to two main/likely things:
The atomizer cut outs where it connects to the battery, or the internal cart. Basically, the inner cart holds itself with "fins" against the walls of the mouthpiece. Sometimes the odd cart will have just the right shape to make a bit of a whistle. Sometimes even loose
e-liquid is enough to give it a little bit of interference.
You can pull the internal cart, dry the outside of it and give the fins a couple brushes over a piece of sandpaper to correct it, or just pitch the $0.60 cartridge.
The atomizer is a little tricker, and would require a bit more work to cut the holes larger, and you'll dramatically "loosen" the draw. Some people actually suggest covering some of the holes to tighten up the draw like a very well packed analog.
You can try covering one or two with you fingertips as you puff and see if it improves the sound, then put a little piece of tape across it for a more permanent solution.
Of course, there is also the possibility that the sound is coming from the internal shape of the coil in the atty, but that seems to be less common. If nothing else works- pitch the atty and get a new one if you cant take the whistle.
