Here´s an answer to the same question, maybe it helps: http://www.italki.com/answers/question.htm?QuesID=12000

The letter r, is easy, if you have audio because I can teach me if you teach your language and good English, contact

TRY TO IMITATE THE SOUND OF A MOTORCYCLE.....INTENTA IMITAR EL SONIDO DE UNA MOTO......

The "R" for a non-native speaker could be a litle difficult to pronounce. Because this is a phonetic problem you have to listen to someone sounding it. I could help you. I am a spanish speaker.