[C320-list] Engine vibration at low RPM. 3GM30

[Graeme Clark]

I used to share a 320 with a Westerbeke for four years but now have my very own with a Yanmar 3GM30
Fitted new mounts over winter and had the shaft aligned professionally. She runs sweet as a nut down to about 1000rpm and then a pretty rough vibration starts up
Is this normal for this engine? If not, any suggestions as to likely cause?
Thanks
Graeme
#366, Jaskar
England

Sent from cell: please excuse typos etc.!