Okay, maybe you have turned off the head bounce thingy, where it may be not as obvious, but:
I happen to be a physicist (for real, it's not just a random claim
) and the thing is that the engine has its resonance frequency just like anything else. So, if you have tuned your engine badly, it will be go into resonance, causing it to shake and vibrate. And, because the engine represents a large mass, it will shake up the entire airplane with it. This, in turn, includes cockpit instruments and, since the the instrument needles are allowed to move relative freely, inertia causes them to bounce as well.
Now this is less obvious in more modern cockpits, as advances have been made in mounting the engine(s) with dampening elements. The engines are designed to run more vibration-free, too and cockpit instruments, if not already digital, have also been improved, "decoupling" them from airframe vibrations in better ways. It's shouldn't be like 1940 anymore
I'm more and more amazed at the level of detail the developers introduced into CoD and it makes me sad that many of those details are, if not quite convenient, called "bugs" by some users. Don't get me wrong, there are bugs and there are quite a few, but CoD is NOT an arcade game and I really, really hope that Oleg, Luthier & co. stand by their philosophy in striving for realism - as this is the "perk" why the majority of players have fallen in love with the Il-2 line.