I get in my car this lovely sunny summer morning, and start it up only to be greeted with a very strange noise coming from the engine. It sounds like a rattle, almost (but not quite) like a diesel sound on a cold winter morning. Think Clarkson's sub-10k "supercar" tappety rattle but nowhere near that loud. This rattle increases with engine speed and doesn't seem to matter if it's under load as it does it when coasting downhill in gear. In addition to that, there's a whistling noise when the rpm needle is between 1000 and 2000rpm. Again, it doesn't matter if the engine is under load as it does it in neutral or coasting downhill. I can hear both more clearly inside the car rather than under the bonnet, but it definitely sounds odd.
When first starting the car, the rattle isn't there but then appears after around 2 seconds of idling. Before you say, it's air conditioning - it's not, as it appears before the familiar "click" to engage air con. Also, does it with air con completely off.
Otherwise, there's no loss in power, no smoke and no alarm displays on the dash.
Any ideas? I do 90 miles a day driving (mainly motorway), so don't want the engine to blow up enroute. Could it be something simple, like an aux belt slip? Not sure why it would generate a rattle though...
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I normally drive between 80 and 90mph with rpm needle sat at around 2800rpm.