Sissling when making espresso

First of all, do you steam your milk first then immediately brew the shot? If so, that is your problem. When you depress the steam switch, the temperature elevates way past what is optimal for brewing espresso. If you try to brew immediately after steaming milk or using the hot water feature, that would definitely be why your espresso sizzles. Your shot will likely come out burnt tasting, thin and just plain rancid.

Either brew your shot first (learn temperature surfing for your best shots), or immediately after steaming milk, turn off the steam switch, depress the brew switch (without portafilter in place) for a few seconds until no steam comes out- only water (wait till you don't hear the sizzling sound). Hope this helps.