Sounds like mice in exhaust when cranking = bad fuel filter?
 

I left it overnight and the tech heard it with this morning, when you crank the car before it starts after it's sat 4+ hrs @ <50f it makes this crazy noise from the rear like mice in a hollow tube... he said he heard & felt it on the fuel filter (which was replaced by a different dealership about 6mos ago)

anyone ever heard of this?

Fuel filter? Never heard of that. I've heard that noise in a bad cat though.

Sounds like an exhaust leak to me.

not exhaust leak, it's like a...
 

"dook dook dook" as the starter turns, loud in the cabin originating from the rear of the car... only when car is cold tech was like "i've never heard anything like this in any car before"

Possible....
 

caused by the fuel pressure regulator inside the filter, a spring loaded relief valve, releasing fuel back to the tank. Your specific filter/regulator may be "unique" regarding the spring tension or valve disk, etc.

they're saying a previous dealer installed it improperly, they reinstalled and will test

fuel filter replaced, noise gone... but now burning smell from vents (more)
 

They also replaced a donut gasket at front of exhaust since they created a leak there on previous service dropping transmission & replacing input shaft seal, lumbar's broken again for the third time upon pickup and now i have burning smell through vents after it runs for 3-4 mins and left idle

*sigh*