theres an issue on zetec engines theres a component iirc in the air system that over the years gets yucked up and needs cleaning with carb cleaner bit i cant remember the specifics edit iirc idle control valve that can cause idling/low rev issues. as to 5th above 35 depends on the car my last 3 did better mpg at that speed in 4th or even 3rd and i would be tempted to say the strain on the engine is more damaging long run
Hiss could be a hole in turbo pipe.
I replaced mine at 93K on a 57 reg Focus. Easy to see/fix
Don't think his car has a blower.