Unusual vibration at idle

[Craigjm]

My facelift has aircon that has not had any gas in it over the winter. Went and had it regassed and a dye put in so we can see if there is a leak and the aircon is working as it should. After that the car has steady revs at idle but there is a vibration that comes and goes. What could cause that? I mean the may not be related and it may be engine mounts or vac leaks but just wondering if it's something obvious