My son is reassured by humming as he falls asleep. Sometimes shushing sounds will do the trick but some nights he prefers humming. I just grab the first tune that pops into my head and slow it down to lullaby speed.

I have hummed the following songs to my son at bedtime:
O, Canada
The Happy Birthday Song
Theme from Hockey Night In Canada
Here Comes The Bride
Smells Like Teen Spirit