It wasn't wind noise - it only happened on rough road surfaces and the noise was like someone was trying to get in! It creaked and rattled and then banged as if it were bouncing up and down. When I stopped and pushed the corner of the sunroof from outside, that also made a noise. My guess is that the ice filled any space allowed for natural movement and so normally silent movements were amplified.
But a scary experience none-the less. I thought I had a serious problem at one point until I realised where it was coming from.
Hopefully a quiet drive home....
BTW - you're not really waiting to have a go are you