Well funny you should ask! I had a whistling standpipe as well and tried a few things i.e. cleaning it, pouring some water through... In the end out of desperation I put the toothpick (I kid you not!) that I used for cleaning over the hole so as to not cover the hole but so the whistling stopped. The idea was to break the airflow going in and thus stopping the whistling sound. Left it there, few days later it got bumped and moved off the hole and lo and behold, no more whistling! I have no idea what caused it or why it stopped but it did.
So maybe not a toothpick but a anything that will still let air through but breaks the flow of air slightly should do the trick.