Posted on the 'other' forum but suspect all the real techies are here nowadays
Just noticed this today, at all RPM's the engine is as you would expect, smooth, quiet etc, and rough running on INPA doesn't show anything out of the ordinary.
A small amount of throttle in neutral to raise the revs slightly causes a lot of vibration as the revs go through 1200-1300 RPM and then disappears. I can feel it in the cabin more than next to the engine, though it is there.
I know these engines have a crank vibration damper, could this be related to that? I did think engine mount but seems odd its consigned to a very specific part of the rev range.
Secondly, at the engine, I noticed a strange new hissing noise when I blip the throttle open - didn't notice that before. I have a new seal for the DISA, might be worth taking that off to make sure its flap is still secure?