Thanks for the responses chaps. Yeah, it's a weird one. The only other thing I can think is that I'm overwicking because I'm using 50/50, and the oversaturation is causing it. I thought I had stuffed enough cotton in there (much more than I ever would with a Kayfun, for example, but maybe it's not enough) Luckily it's a super forgiving RTA to wick, so I'll try stuffing some cotton in the "cups", as well as the deck itself and see if that makes a difference.
@Mr.Motivapr - Depending on how you like your DTL draw, I'd be slightly wary. It's an amazing RTA, but even on it's widest setting, with the "DTL" deck, it's still what I would consider a restricted DL. Ironically, I never get whistling when using it in DL!
Also,
@Rob /
@mattyk2010 - How high are you placing your coil? I'm using 2.5mm id with the top of the coil about 1mm above the top of the posts. It seems to have helped slightly, but it's definately still there. Previously I was at, or just below, the top of the posts.