I scrape my foot on the ground to make a noise

I can't help it. I politely say 'pardon me' then they look around and need to change their underwear.

Don't worry about it, in this day and age there are plenty of videos of people walking into ponds, signposts, whatever because they are not paying attention to their surroundings, being startled is a very mild outcome.
When I was about 19 I was off work sick for a while and while I was lying down resting somebody tried to break in, I bellowed at them, they scarpered so all was well and good. Anyway a couple of days later still off work and hear someone opening the garage door to come round the back, so I thought the same p&^)k was back again, go into the kitchen, grab a chef's knife and out I go to confront them yelling loudly. Problem was it was just a delivery man nobody had bothered to tell me about, poor guy was more than startled and I apologized profusely, however he got over it once I explained about the attempted break in.
The moral of the story is that being startled is not likely to hurt anyone, not being aware of your surroundings on the other hand can be dangerous. If you've said hello in a low voice I'd say no fault: no foul, though I would suggest that a normal speaking voice might be more appropriate as people whispering at you is actually more threatening (seems like they don't want other people to hear.)