I’ve been having this problem since i started to use VSTs more often. Everytime i add an effect to a guitar/piano/vocal, i get a random hissing noise for 2 or 3 seconds while the recording is playing or even if it isn’t, also, the hissing doesn’t happen on a specific part of the recording, it just goes off at random. I have added the VSTs directly to the track and i’ve used FX channels to try to fix it, but it still keeps happening no matter what i use. Does anyone know how can i get rid of this? 
Hi and welcome,
Make sure, you are not using any plug-in in DEMO or Trial mode, please. Some plug-in vendors generate noise in this mode.
How can i make sure i am not using them in Demo or Trial mode?
Hi,
The only one way is to load them and check. But it cannot produce the noise of it’s not loaded. So you can exclude plug-ins you haven’t used in the Project.
In my current project i am using an amp simulator and a reverb effect, both directed to the guitar track via an FX channel, i don’t really know if the pre or post fader option can influence the hiss, but, as far as i’m concerned, both effects are not demos or trials. Also, i don’t know if the amount of effect i put on the track can make the unwanted noise, could it be possible for the hissing to go away if i change an effect from 0.00 to a -15?
If you are using Steinberg plug-ins only, then this is not the case.
Can you hear the same when you export the Mixdown?
Did you try to increase the Buffer Size? Isn’t it just distorted?
I am not using Steinberg plug-ins only, but i haven’t tried increasing the buffer size.
I put it on the minimum to be able to record my midi keyboard without delays, i am guessing that i must increase it after recording to make sure the effects work well, right?
EDIT: Just tried increasing buffer size to the max, the hissing is still there.
Did you already check all plug-ins if they are not in the Demo/Trial mode, please?
For sure there are some plugins in demo mode.
Hiss + periodically = demo restriction.
Also it’s really annoying when you describe plugins as “some”
Nothing is easier than to just remove / bypass plugins 1 by 1 to see which one is causing it.
Found the problem, a Reverb plug-in has an expired license, thank you for your patience (Sorry, i’m kind of a newbie using VSTs) 
I found one with an expired license, pretty sure that’s what’s causing the problem, thank you for your patience (i’m kind of a newbie on VSTs, sorry)