HELP! Squealing front wheel.

My front wheel has developed a high pitched squeal at very low speeds ie when setting off or stopping.

I had the bike on the stand last night and here is what I've worked out so far:

1) It's definitely not the brakes
2) It only happens when the QR is tight (ie I can almost make it disappear if I set the QR looser but not really an option)
3) There doesn't appear to be any play in the wheel when the QR is tight, or at all for that matter
4) It only happens at very low speed
5) Although it is clearly the wheel it almost sounds as if the noise is coming from the juncture of headtube and fork?

The wheel is otherwise good, no grinding, spins freely and although it is very slightly out of true there isn't anything else wrong with it that I can tell.

Any ideas?