Assassin's article on tracing vibrations is very interesting and as he says tracing a static vibration is easier than tracing a moving vibration.

In tracing 'static' vibrations the offending items need to be 'excited' at their resonant frequency, most easily by running the engine and revving it up to generate a range of frequencies.

If the vibration occurs with trim inside the vehicle or under the dash and running the engine didn't readily reveal the problem, you can use your vehicle audio system in diagnosis. If you have access to scientific equipment such as a signal generator you could feed exactly defined frequencies into your vehicle audio system and find the exact frequency at which the offensive resonance occurs, leave that frequency running and hunt down the problem.

Most people (including me anymore) don't have access to such equipment but I read about an alternative method not too long ago. You paid a tenner or so for a cd which when played through your vehicle sound system generated test frequencies with the express purpose of exciting vibrations and rattles. The company said they could provide me with an audio cassette of the same thing.

The really good news? I can't remember who the company was but for those in desperate need I'm sure Google would provide the answer.