Mickyg, yours is a petrol V8 I believe - auto or manual? Can you pinpoint noise is from the back, mid or front?

As said earlier, petrol tank.

Could be partially blocked breather tubes or seals sucking in as things cool. Brake booster losing vacuum - head in footwell and sense noise coming from brake pedal area. Not getting flat batteries? - failed diodes can 'whistle', but is ongoing until battery goes flat, and is faint.