Vacuum Noise in 5K

[Steve Sherman]

Hi.

My 5LTQW has an unusual noise, whenever I let up on the gas pedal.  It
sounds like a faint vacuum leak. I am still on the trail of this elusive
noise.  It happens after running the engine at power then letting all
the way off the gas at once.  A low volume hissing noise comes from
under the dash and last for a few seconds.  I'm guessing this is a
vacuum leak, because of the sound and that it happens at throttle close
which should spike the vacuum.

Unfortunately I have not been able to find this leak (if that is indeed
what it is).  And the noise does not happen at idle/stationary so I
cannot reproduce it with my head under the dash listening.  I suspect it
is a pinhole or other small leak and so likely to be hard to spot
visually.

Any thoughts of likely places to look, or other problems to look for?

TIA