Thats an interesting comment... I think I have had a similar noise, but very rarely and only under full acceleration. The one time it happened as i accelerated through several gears.
As you say, behind the dash and a sort of sizzly noise... I thought it sounded like aerated liquid being sucked at high pressure through a pipe or something.
I thought Fuel line as it seemed to happen under heavy acceleration?? but surely that doesnt come into the cabin... odd.