Welcome to the site Portista. Hope we can help.
I would say not. I'm with you on your assessment of what the noise levels should be like. If the amp is brand new and under warranty I would take it back to the shop and demonstrate it to them. If it came through the post I would contact H&K on their Facebook page, (emails are hit and miss, FB is more public). They are pretty good at responding on there.
H&K Official Facebook Page Be careful there are a few non-official sites out there under Hughes & Kettner!
If you want to look into it yourself the simple things you can do are... Try swapping the two preamp valves around, that's V1 under the little metal can and the one next to it. If the nature of the noise changes, if it maybe appears on other channels too, then you have a noisy preamp valve. If you have a spare 12AX7 in maybe another amp then swap each of them out with that in turn. That would either show it as a preamp valve or rule them out. As I said earlier, keep a note as to which valve fits in which socket and replace them back in their originals when you are finished! One tip, number all of the valve tops near the nipple with an indelible felt tip marker to be sure to put them back in their correct place at the end of testing. Despite the myth you will come across by internet experts it does absolutely no harm to the valve, it is NOT a halogen bulb, just a plain hardened glass tube!
There is a proper procedure for opening the amp up to change valves. H&K like everything to be done by "qualified personnel" like any shop tech who could wander in off the street and blag a job, (not ALL techs by any means, the good ones are great and diagnose and repair things at a speed I couldn't which is their job), not bodgers like their owners so they put anti-tamper measures in place. (Anyone remember Flanders and Swann, It all Makes Work For The Working Man To Do?) You MUST NOT take the end caps completely off, that breaks the anti-tamper tabs inside and voids your warranty. On both of the caps at the ends of the amp remove the two top screws and the rear handle screw completely leaving the front handle screw alone, it does nothing except holds the handle onto the cap. Then loosen the bottom two screws by only a turn or two until the end cap can lean out enough to slip the metal cover up and out. Don't worry, there is a lot of spare play in these screws so you shouldn't get it wrong. When you refit you have to be careful with the LED strip which sits on top of the acrylic front panel. It is held in place by 5 metal clips which must be seated correctly before the lid is put back on. It's not hard, you just have to keep your concentration.
You are not putting yourself at risk here, there are no exposed electrical parts in the top chamber of the amp. And you can briefly test the amp with the top off as long as you keep fingers and tools out of the valve space.
Now I have to give all this advice based on the GM36 which is the GM40D's older sister. They share the same pedigree but we have to assume that the GM40D has the same circuit configuration as the GM36. It may not! despite that, this won't do any harm at all and it should show up a preamp valve problem.
I wouldn't have thought that this would be caused by a power valve EL84 but it is possible that H&K reconfigure the power amp feedback or voicing for each channel and that might be a factor. For the Power amp EL84s, does this happen with the Power soak in the 20W position? That removes one pair of those valves. If the preamp 12AX7 valves show as ok, that is no difference when swapped, put them back in their original places and try swapping an outer power valve with the inner one next to it and retest at 20W. If they are ok then you want the amp undisturbed in that area. If you find a noisy valve you MUST replace them as a matched pair, the outer two work together as do the inner two and should be matched.
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