EVP's seem to all have the quality of the slurred word, this might be because we breath words, spirits do not breath, as they don't have a physical organism to breath with. just a thought. I've listened to a couple Art Bell EVP shows..quite remarkable, some of them.
perhaps spirits are learning just as much how to do this as we are learning how to receive them? I don't like calling them spirits, we can call them nonphysical persons is better.

although you could argue the whistle must have air to produce it..so hmmm...I do know their frequency vibration is faster on this dimension, as when I was receiving thought vibrations, impressions from my grandmother once, I had to ask her to put a delay on the transmission output to me, as I was receiving an overload from her end, and too fast.

here's a link to what you can try to contact grandma, I tried it and it worked the first time I tried it, but you may need several tries at it, as I found the communication extremely subtle, and a bit difficult to keep the doubts away that you are "really" communicating..however, practice does make perfect.

http://www.afterlife-knowledge.com/articles.html

when u get there click on Appendix C; it's an article, not very lengthy, you can get a general idea and try it.  let us know if you are successful. love, alysia