Car running somewhat better with a new cat but its twice as loud. Mechanic said it sometimes takes a while to adjust. That doesn't sound quite right.
My guess its an aftermarket part as it only cost about $300 before labor.
Or he just put it in wrong. It revs like a jet engine when accelerating. Any ideas for my next step?

And, 2005 xB 230,000 miles. Was having a lot of cylinder misfire probs previously. Not sure even that was taken care of correctly.

 

There's no reason it should be so loud - if everything was installed correctly. For $300 in parts, I'm pretty sure it was an aftermarket converter. Although the converter does quiet the exhaust somewhat, if it's that much louder, something is wrong.

On a cold morning, look underneath the car when you first start it. Look for exhaust smoke coming out anywhere other than the tail pipe. That would be a sure sign of a leak - and thus at least part of the reason for the increased noise. Any codes?

 

First step for cylinder misfires are the spark plugs