Strange, soothing, high pitched tone from outside this morning - London UK?

Hello!

Ok, just thought I would put this out there in case anyone else in the North London/Herts area heard anything this morning 26/08/2011

I woke up this morning and became aware of a strange sound that had awoken me from outside. It sounded like a very high pitched, but mellow constant tone. Almost like someone wetting their finger and running it around a wine glass. My wife awoke at the same time and sat up saying “what the hell was that noise.” - The sound faded and disappeared just after my wife spoke!?

It was quite a nice and soothing noise, not unpleasant. : )