A plugged cat, or one with a big chunk capable of getting sideways (and plug intermittantly) could cause both of your symptoms: lack of response and throaty intake sound.
The ones that turn sideways intermittantly can be a bugger to diagnose. We will cut the pipe right in front of the cat to view when testing concludes this as a possibility. We weld it back when all is OK.
Bosch Master, ASE Master, L1
33 years MB technician