I had the same problem on my iSonic 2007 101 34 cm Drake fin last year which was very annoying. I posted the problem on a thread and was told to slight 'water sandpaper' the back end at a 45 degree angle etc etc but never managed to stop the fin whistling. In the end I went and bought Techtonic fins and solved the problem once and for all. Bottom line is that the Drake fins are second rate fins in my opinion should not be supplied at least with the iSonic line cause they are not good for the job. Basically it's like buying a Ferrari which is supplied with tyres suitable for a tractor if you catch my drift ha ha.