Diggin' up a fairly old thread here but I found this today.
Here's something to consider:
"Exposure to noise at night can suppress immune function even if the sleeper doesn’t wake. Unfamiliar noise, and noise during the first and last two hours of sleep, has the greatest disruptive effect on the sleep cycle."
Here's the source: http://abc.net.au/science/sleep/facts.htm
Looks credible enough to me.
Therefore, sleeping with a metronome might not be the best idea.