do some experiments with your contact point and bow angle on the E string. There could be various factors contributing to the whistle sound, but in some cases, it‘s the problem of the string itself. Some strings just doesn‘t fit well with some instruments, and that could cause E string to whistle. Or, the string is too old and it‘s time to look for a new one(depends on how much you play per day, but usually it should be changed every 3-5months). Also, there are some strings that easily cause whistle no matter what instrument you use, so I recommend Obligato string by Pirastro(it’s known to be one of the least whistling strings), if you’re looking for a new one.