Rebuilt it on my lunch break, installed a new coil and moved it up and packed in some more cotton. Made 100% sure that there was no cotton over the airflow channel. Still whistling It's weird that it can be so quiet when doing DTL but as soon as I go to MTL it whistles like a bugger! It's not a high-pitched whistle, just more of an "air being sucked through a tunnel" with a bit of whistle in the background.
Will re-try the 1.8mm deck later and see how that goes, but now I've noticed it it's bugging me more and more :-/
Also, @Rob - Removing the airflow ring does help, but it's still there, just not as loud. I've installed the airflow ring both sides up and it doesn't seem to make a difference.
I now have 3 MTL and also a 2.7 deck and depending what combination of air ring and size hole I use get more of a typical air ‘whoosh ‘ bordering on hiss occasionally but never a whistle.
I use a 3 mm coil again always in TC, my usual build is in the ‘Let the battle commence’ thread, further down.
As Rob says, best to make contact. If you have both size air hole rings, try the one that comes with the 2.7 deck as for some reason I find that quieter, although if you get whistle without any attached probably won’t help.
I dont get any whistle at all but have you tried taking out the juice ring. I dont use the juice control ring its always full open. Mine is left in place but it can be removed. Might help