This is a problem with me in spanish... I can do a uvular trill (rolling the r's with the uvula... the hangy thing in your throat that if you touch you puke). But in spanish, the uvular trill is never used... I can do a crappy version of an alveolar trill, but I dont think I am doing it right (my tongue vibrates a little, but its more of a fart noise... and when I talk while doing it its more of talking with a background of a fart noise).