I think a clay bar might do the trick... It's probably some sort of environmental crud that deposited on your finish...
They're available at your local auto parts store such as Auto-Zone... Meguiars makes one... and here's another type for more details on what clay bars can do...
http://www.clay-magic.com/ http://www.clay-magic.com/whatisclaymagic.htm "What is paint contamination?
Paint contamination consists of tiny metal shavings from rail dust, brake dust and industrial fallout. This contamination affects all paint finishes and can cause serious damage when left untreated. Paint contamination can be felt as a "rough or gritty" texture on the paint's surface and can lead to tiny rust spots. This contamination can not be removed by washing, waxing and/or polishing.
Where does contamination come from?
There are three major causes of paint contamination:
1. Rail dust - produced from the friction of train wheels against railroad tracks. Over 70% of new vehicles are shipped by rail. Rail dust can contaminate a new car's finish before it even reaches the dealership. Anytime a vehicle is parked or travels near a railroad it is subject to rail dust contamination.
2. Brake dust - particles produced from the friction of brake pads rubbing against the rotor. This metal on metal friction disperses tiny particles of bare metal into the air and on the highway where it collects on passing vehicles.
3. Industrial fallout - another word for pollution, industrial fallout is a byproduct of our modern industrial age. "
A clay bar will take care of these things...