Volvo: volvo 850 GLT Wagon 1997, volvo 850 glt, pep boys

Question
What makes the car have a low rougher sound at idle, instead of a smooth/steady quieter sound like I used to have? There are no codes showing up.
Thanks!

Answer
Does the car run the same in all other circumstances?  If so, it could be that your idle is simply set too low.  Also, check that your timing is set correctly.  Finally, if the car doesn't run very well all around, you could have some gunk in your fuel injectors.  You could try a fuel additive you can buy at Pep Boys that cleans the injectors.  Also, check to make sure you don't have a clog in your fuel filter or your air filter.