I had a similar issue with a new Dell Arte Boulou Ferre model.
When I got it, it was grand, except there was a sympathetic vibration
coming from the soundbox when I hit certain notes on the "D" string and
if I recall the "A" string.

Although I didn't have the proper wrench to play with the truss rod, it
seemed as if I could wiggle it with my finger.

I sent it back w/i the 48 hour approval timeframe, as I went sour on
this little annoyance.

Maybe have a look at the trussrod.

Just a suggestion,

Ernie