It's the one where elliot is going out with sean but he decides to try to do this long distance relationship thing and the song starts when jd is in like the hospital lobby and dr cox says to him he can have the night off so he goes outside and the song stops and he talks to the janitor but then it starts again and he goes to elliot's house
If you can tell me the season number and episode number (find it on the episode guide at tv.com), I can find the soundtrack.
Or better yet, find the clip on youtube and link it as an answer, I might know the song off the top of my head.
