If she is new to clipping, then it is probably not the sound, but the feel that is freaking her out. I'm sure it tickles something fierce. I don't know, I've never buzzed my head.
*There are several ways that we at our farm desensitize ours to clippers. The main way is the clipper stick. We have a set of clippers that do not clip but the motor still runs and makes normal loud clipper sounds taped to a stick, and we just work with them in thier stalls everyday. We work a little at a time, just seeing how far we can get, first desensitizing the air, and eventually working up to petting them all over with the clipper stick. Down the back, on the shoulders, down the legs, up the neck....everywhere. Little bits at a time, what we can get for free. Unless we are planning on showing them soon, we aren't in any rush.
The reason the above clipper is taped to a stick is so that if the horse, who doesn't know yet what is going on, gets defensive and strikes, you are not going to be in range.
*One of my favorite tools, because it is cheap, lightweight, and very handy, is a Pulsar Toothbrush. It makes a nice buzz, it vibrates tickle-ing-ly and you aren't going to want to kill your horse if you break it while working with him. It can easily get in and around the ears, you can play around and try to brush their teeth, lots of fun. I need to go find mine.
*The problems with distracting with food that I see are: A) you can't get to the muzzle; B) If you sneak up and surprise him with his head in a bucket, that head is going to fly up and might smack you in the mouth. OUCH!!!!! *insert expletive here* ; C) Food may run out/ may not be available/ the horse may get wise and/or the effect may wear off.
*Sedatives are often used, I've found, but in a lot of cases, do not work. They tend to make them sleepy, but thier ears still work, and so though they may be drowsy, you are going to get about as much done as without them. In SOME cases of clipper training, mind. I know some trainers who will lightly sedate the first so-many times they clip, and supposedly they claim that as two year olds are perfectly mannered clippers.
*Something you can do, or have done, to help ease with bridlepath clipping: With someone holding your horse, if he is good with the sound and so forth, have that person just put his/her hand behind his eye, like the blinkers on a driving bridle. Don't cover the eye completely, just block what is going on behind him.
When I was getting ready to show my yearling in hand and it came time to trim him, he tried to get scared and silly, and mom just blocked his view, and he was clipped with no problem.
*I'm usre it is common sense, but we never cut the hairs around the eyes. I know a lot of people who when they go to a show, EVERYTHING comes off. We always leave the foretops(for looks) and the eye hairs(for safety).