911SC Odd 'sizzling' noise when step gently on gas
 

In my 82 911 SC I have noted a subtle sizzling like noise when I gently step a little on the gas, only when the car is moving. The noise is coming somewhere from within the car. It does not matter what speed I am going, nor what gear. If I put the car in neutral while travelling, there is no noise when I step on the gas. Any idea what this could be? It sounds more electrical than mechanical.
Thanks for suggestions.