Well, my car suddenly started making a sound like if the catalytic converter was removed, which it has not. The noise started when my husband connected the a/c compressor directly to the battery because it didn't turn on automatically when we turned on the a/c on the dash. So, yes the compressor is on all the time until it gets fixed and we already know it is because of a bad transducer. So, with that said, is that why my car sounds like it has no catalytic converter?
I would start by getting down low/underneath it, on a cold start and looking under the car to see if there's a visible exhaust leak.
Have someone rev the engine a bit, and see if you can see any puffs of smoke.
Shining a flashlight on the likely areas can sometimes help, as well.
If the sound isn't as extreme, it might have something to do with an air silencer in the air intake "System". There's a possibility it was jostled out of place during his working on the vehicle.
I hope this helps.