Guys,
I used to (And still do from time to time) do this on several aircraft, ranging from Helicopters of all shapes and sizes, Robins, BAe ATP's and even one rather lovely Dessoutter from the Shuttleworth Collection... Obviously I didn't shake that one very hard!!!
The method of vibe testing, as Genghis has already mentioned can involve a bonk test, although I personally use a shaker. Basically we Jack the aircraft into a level attitude, gear up, and attach the shaker. The frequency and amplitude of the induced vibration is controlled via and amplifier. I use real time FFT analysis, although I have been known to rig up oscilloscopes and function generators etc.... too much of a kerfuffle in my opinion.
You didn't really say why you are doing the vibe survey? If its a design function then your approach will be very different to mine, my methods are used purely in tracking down vibration related defects on in-service aircraft.
DE