You need to reproduce the sound.
Does it happen with the engine off but ignition on? Could it be a loose fuel line moving around because of pressure change? Just a thought.

This reminds me of two things... once I ran over a raccoon on the highway and it hit the car right where my feet were.... gross.

Once I bought a 1983 BMW 633 with a 5-speed (AWESOME CAR TO DRIVE!)
And the steering box mounts were broken so the previous owner used big U bolts to crank it down. It shuddered terribly braking at highway speeds.

This car also had bad camber in the rear, so the tires were worn to the steal belts on the inside. When you pealed out the car threw a shower of sparks... Again... Awesome!