Hi,
Based on my Dixon older models, they have a hallow fipple/stopper. I have also made Tony Customized my DX202 Head in Similar to the Trad Model.
If the case is hallow fipple you can put a tacky poster putty instead of toilet paper.
Toilet paper accumulates moist and can be a breeding ground of bacteria/yeast/fungi/virus/amoeba/protozoa/algae,
etc.
If you came from a Recorder/ Classical Flute Background, you will definitely comment and notice the Hissy E (Second octave, Second Note) of a whistle.
I also noted that on my Susato in all keys, Feadog, Generation and Dixon on my first try.
Eventually I have learned to embrace the quality/bugs of a tin whistle.
Even a sweet and pure birdlike Bluebird Freeman has the same hissy sound (in my ear)
My description of the hiss is it is kinda mute the note a little (but still louder than Cnatural), and you hear in your own ear the “F” Consonant.
If I will phonate/enunciate it, it is like pronouncing ‘efffffff’ (like “elf” without “L” but “F” is somehow sustained). If this is not your description, then disregard this message.
I think, IMHO, it is based on your experience that you are used to the ‘clean’ and somehow smooth and quiet sound of a Recorder in second octave or sweet sound of metal Classical flute Flute in comparison to the overblown ‘Chiff’ of the whistle.
However I tested this theory if it is my ear or the instrument.
I made a recording of my own tune practices and
apparently the sound becomes clearer and no hiss was recorded.
My theory is, it is my/your ear.
The sound we hear when we play a tin whistle and flute is actually 2 sounds.
First sound: Sound of the notes played
Second sound: Sound of your breath/air traveling to the tubes/traveling to your cheeks then to your ears.
Your own ear is very much sensitive to even hearing your own breath. If this is the case, you also have a good ear/paranoid ear 
*Notice that the whistle has 6 hole and E+ note is the first note that is only one hole that is open.
Where the B3 hole serves as a vent.
*Notice also the velocity of the Air on the second octave which is faster than that of the first octave.
When your air velocity is faster and you have a smaller vent, you normally hear a hiss (my definition)
In my own definition (not this is my OWN, you can have your own definition), I think it is a Venting effect.
I define the venting effect as a force where air exits in a narrow hole.
Notice that the sound of F#+ has no Hiss (my definition) because air has vented on 2 holes already.
I happen to notice also that this venting effect is somehow minimized if the B3 hole is larger.
I am not good in Physics, maybe Hans can comment on this. He is a freaking master on this.
Hans, we need your guidance. 
Regards,
Angel