Cuba transmits noise on the same frequency as Radio Marti broadcasts. When I listen to 6030 khz where I am, I hear a sound like running water on top of the broadcast. Here's a recording of it. This Radio Marti broadcast came from a Greenville, NC transmitter.
6030_khz_13mar2006_0215utc.mp3 - 2 min, 486 kb
Also see my previous article, is radio marti propaganda?