Hello all, new to the forum here, this will be my first post.
I recently purchased, at a bargain, a Stanwell Sixtus 211, cutty/dublin shape pipe (the bargain came because this line is now discontinued). I fell immediately in love with its looks and when I took it out of the package, was amazed by its lightness and comfort in my teeth. It's seems like half the weight of my Peterson. And the grain is quite lovely. All in all, it's flawless, as far as what I was looking for...almost.
It whistles. My Pete does this too, mildly, and only when empty, but the Stan whistles like a tea-kettle when empty and can be faintly heard when filled (filled only 1/3 of the way, that is, I'm just beginning to break it in, only two smokes on it yet).
I've already read some comforting comments on the topic of draw width, that it's not necessarily evil, but I am having some difficulty with it so far, keeping it lit and burning cool and so forth. That is to say, it burns surprisingly dry with my amateur suckling and cools down speedily when I let it sit (it breaths well, I suppose), but should the whistling concern me? And are there any...mild solutions?

They might be some air coming through a gap where the tenon meets the stem. Since it's a new pipe, my immediate reaction would be to do nothing at first during the break-in period, particularly if all other things are functioning well: the heat of the smoke might expand the stem somewhat and solve the whistle (I've had loose stems cure themselves this way). Then go onto extreme measures, if necessary.

If there's a flaw in the drilling, it can create a vortex, causing a whistle. Might be the stem or the bowl. Take the stem out and blow through it see which.

Stop letting pretty women walk past your pipe--the whistling ought to stop.

I see you note that you are still breaking it in and only filling it about 1/3rd of the way up. If it isn't an issue with the drilling, it could just be that your pack is too loose and there isn't enough tobacco in the bowl.

Sounds like the airway needs to be opened slightly.