Loud whistling sound from front of engine

So I had a problem with my ST1100 which I solved by installing a new battery. Alright, time to ride again! So I do. I get about 25 miles from home and pull over to get some gas. I fill up and before I put my helmet on, I decide to start it up. It starts up and immediately there is what I can only describe as a loud whistling sort of noise come from the front of the engine. Turn it off, noise stops. Start it back up, noise instantly resumes. I rev the engine. Very little change in the pitch of the noise, though it seems to become slightly less loud. Grrr! So I ride back home and once again I'm back to working on it rather than riding it. Good times.

So I just started it up for the first time since I got back. No noise. So apparently it only happens when the engine is hot?

Whistling is a new one. I would check for trash tangled up in the radiator fan, leaky or cracked water hoses, radiator cap loose or with cracked seals, sticky starter. Assuming you have a constant whistle. Pulsing whistle may indicate a loose spark plug.

Possible leaking exhaust header gasket.

whistling, as in steam coming out of a teapot? Maybe your radiator cap was blowing out some steam.

Or if it was a high pitched mechanical squeal, it could be the water pump bearing going bad.

That mouse in the air cleaner not happy his home is being moved?

Sounds like the fan may be running when the noise is present. There isn't much room between the fan blades & radiator. Many something is caught in there.