Whistling sound

I had a weird whistling sound coming from the old cabby today and I was hoping for some ideas on what it could be.

I'd been driving along the motorway for about 45mins so the engine was hot, I pulled up into traffic and a really loud whistle sound came from the engine after a few moments, similar to the sound a camping kettle makes when it's boiled.

I'm thinking either the fan rubbing (looks ok now though), fan bearings, or cooling system leaking/letting off pressure (but it doesn't appear to have lost any coolant). Or possibly a belt slipping or bearings whining maybe?

After about 2mins of stop/start traffic the noise went away.

Any ideas?

could be any of those things really, i think you'll have to do a bit of investigation