I’ve done a lot of door knocking for political campaigns. Technically, we aren’t salesmen and the “no solicitors” sign doesn’t apply, although the odds of a positive response from someone with such a sign are low. 
I sometimes rang the bell, sometimes knocked “rap, rap”, and sometimes did “shave and a haircut”. Then i stepped back so as not to be close enough to the door to look threatening. (I’m an older woman; I don’t look very threatening. But no one wants to feel crowded at their door.)
One of the people who trained me claimed the “shave and a haircut” got a better response, because leaving out the “two bits” made occupants a little curious. But honestly, i mixed it up as much to alleviate my own boredom as for any other reason. But if there was a working bell (usually, there wasn’t) i started with that, because i feel like that’s my preferred notification at my door, so I’d guess other people prefer it, too.