How can I quiet a bathroom exhaust fan that became loud only after the duct was added?
There more restricted the airflow is, the more noise there will be. All contributing factors to noise:


Diameter of duct (larger is better) 
Overall length (shorter is better)
Number of turns/bends (fewer is better)
Radius of turns (larger is better)
Size reducers used (no reducers is better)
Type of duct (smooth, rigid is better than flexible) 
Type of exterior vent hood (low resistance opening is better for noise)


This is why the low noise fans often have very large (eg. 5") duct connections. Larger duct is more expensive and harder to install.

Note that adding ducting will ALWAYS make the fan louder than no duct, but you just have to accept that because duct is absolutely necessary. Starting with a low-noise fan is a very good move of course (and 0.5 sones is great). 



If you had a contractor install a vent that vented it into the attic, all their credibility is immediately gone, because not only is that not to code, but it's an incredibly bad thing to do: in the worst case, it will lead to mold and rot in the attic space, and ice dams in the winter that will destroy your singles (and also lead to rot and leaks). 

So if that contractor came back and installed vent, I would not be surprised at all if they did it the cheapest, easiest possible way, which would likely be using a 3" (maybe 4") flexible hose, and not taking care to keep it as straight as possible.



First step is to figure out what was done and size/length, etc, and then likely upgrade it to something better. 

The best possible thing to do is have a straight, rigid large-diameter duct directly out the wall to a decent vent hood. 

Pay a few extra bucks to get a decent vent hood that is weather- and insect-proof, but has low-resistance to opening with airflow, especially if it's in a place that sees driving rain or lots of wind. The cheap plastic louvered ones can work if they're sheltered, but they are bare minimum. 

Couple other tips:


Don't allow any dips in the pipe where condensation could pool
It's generally preferable to have the duct sloping towards the fan, especially in areas where temperature goes below freezing
If you have to have turns, then large radius turns are better, which you can make with a couple adjustable duct elbows. You can also mix rigid and flexible duct where it makes sense to. 
Adding a bit of flexible duct will actually help absorb sound and reduce vibrations transmitted from any rigid pipe


The goal is to have smooth even airflow, with minimal resistance and turbulence.