First, I'd check the gauge on another car or put another gauge on yours. That will rule out a bum unit.
Finding a vacuum leak is a real bear of a project. Simple way to start is get a can of Carb Cleaner and squirt it at anyplace that air might want to get into the engine. Hoses, intake manifold gasket, brake booster, etc. If there's a leak, the RPM's will increase a bit.
If the reading is low, you can live with it. Maybe just how it's connected or a quirky gauge. The buzzing isn't too good, might want to check your valves. If one doesn't close properly you'll get a backflash from the cylinder that will disturb the incoming air.
Check out
RWD - The Volvo Owner's Resource
Lots of help for your car.