Have you closely inspected the paintwork for any signs of damage? The chrome surrounding the knobs for scratching?
The other day I had a sound I swore was under the car somewhere - turned out a bonnet catch was not latched. Sounds can be really hard to pinpoint.
You're welcome to borrow Harry - he's very sharp-eared! Mind you, your own son, I'm sure, has better hearing than us.
Metal on metal doesn't sound good - I'd be really checking for evidence of any contact marks. Could the hood frame be 'shimmying' on it's bolts?
