An English speaking person has accidentally fallen through to a parallel world and is now stuck there with no hope of return. English and other Earth-native languages are not spoken there.
As they continue to survive and (eventually) thrive in this new country, they begin to suspect that there may be other people from our world hidden within society: iPods, t-shirts and a few books in English have appeared at markets or as curiosities on display. They didn't bring them, but maybe someone else did.
They have come up with the idea of humming or whistling popular tunes as a low-risk identification method. Which brings me to my question: which songs should they hum to be noticed by the broadest range of fellow travelers from Earth?
Assume the following:
The country they find themselves in is Tudor-ish in terms of technology, is very wary of 'foreign spies' and has a whole bunch of superstitions around magic. This makes speaking or writing a foreign tongue risky: you can be arrested for being a spy, hounded by a mob for 'spell craft' or thought of as mad.
The tunes should aim to reach a broad a range of potential travelers from Earth, so limiting it to popular tunes within a country (eg national anthems) will potentially find less people than something more widely known.