Get the same around here, normally Hercs, Chinooks and the Yeovilton stuff at night and i can say that i am not hearing the Chinooks half as early as i used to when either low or higher up.
Used to hear them miles off and wait for ages to see them whizz by but now it's short notice, 20-30 seconds atleast compared with much more previously.
Not saying that the Chinooks have a different blade fit but it's certainly less than it used to be in all conditions here. The daily A109's of Castle Air make more racket inbound than Chinooks now