My dog goes crazy when she sees a UPS truck. She does not mind Federal Express trucks, USPS. trucks, or those of any other company, but, for example, she will be lying calmly in the middle of the living room floor and suddenly jump up and run to the door barking like crazy and a few seconds later I will see a UPS truck drive by. When we are out walking, she barks at all the UPS trucks and ignores all the others. Do UPS trucks have some device aboard that sends out a peculiar irritating sound that only she (any maybe other dogs) can hear?
UPS trucks probably do have a particular resonant “rumble”, due to their boxy shape and transparent plastic roofs, but I wouldn’t classify it as more annoying than any other truck.
And your dog may associate that sound with strangers coming to the door.
UPS trucks may sound funny because they’re not trucks. They’re officially very large and oddly-shaped cars. (“Package cars” is their right name, which sort of suggests they’re made of cardboard and held together with sticky tape.)
I thought Cecil expounded on this long ago, but I don’t get any hits in the Archive for ups, ups truck, or package car.
Having spent a goodly portion of my life driving one, I assure you they sound much like any other delivery truck I’ve driven (from the inside at least). Maybe your dog is hearing something unique about them (and associated it with something threatening). My dogs are equal-oppurtunity barkers; They bark at all the major carriers (FedEx, USPS, UPS, DHS).
There might be a high frequency sound, audible to your dog, from the air induction system that is unique to the UPS design.
I’m gonna need a cite for this. 
What makes you assume your dog did not have a bad experience with a UPS truck or driver while you were not there? Are you always home? Maybe he just does not like ‘Brown’.
That doesn’t answer the question of how the dog can distinguish between UPS trucks and other trucks.
I have a vague memory of reading something similar (on this board?) where this very topic was discussed.
IIRC, it was mentioned that UPS trucks have a turbo compressor or something unique to their trucks, which creates a high-pitched whine that dogs will bark at. My search came up empty though, so feel free to disbelieve it.
My dog didn’t bark much at anything. Except UPS trucks. And she could hear them coming from it seemed blocks away. I think Telemark hit the mark… The dog associated the unique truck noise with strangers coming to my door.
I figure that’s one reason we have dogs. 
Lots of vehicles have turbo apsirated diesels, from big pickups to truck tractors. They’re not that high frequency-I can detect turbo from non turbo under acceleration.
Animals associate sounds with something they like or don’t. The lady next door has said she knows when I’m coming home a few minutes before I arrive, as she’s seen the cats come out of the woods and gather on my porch. How they can hear my Ford van and differentiate it from the thousands of other Ford vans out there is beyond me, but they do it. 
My old cat knew my footfall when I was walking up the street. I’d be walking home in the middle of the night and Mayhem would come running out of an alley a block away from my apartment and trot along beside me the rest of the way.
He could probably see you.
UPS trucks do have a unique sound. It’s partly that they are lightly muffled, and partly because the body has big flat aluminum sides and a translucent plastic roof that are free to resonate with the engine noise. I can hear one coming a half mile away. Maybe it’s for weight savings, but I have wondered if it’s to let the customer know the UPS delivery is here, so the driver won’t have to wait for somebody to come to the door.
I knew a guy from a small town who would go out to edge of town at 5:00 pm just to listen to the UPS truck go through all 13 gears on the way out of town. 
Well cars/trucks of the same make/model/year/configuration are going to sound similar. Just because average humans can’t classify delivery trucks by sound alone doesn’t mean other predatory animals cannot.
*Can dogs classify cars/trucks based on make/model/configuration better than humans based on sound alone? *
I have no cite but I do not doubt this for a moment. I bet my dog knows exactly whose car is pulling into the garage based on engine noise. There doesn’t need to be something special about the UPS truck noise for your dog to distinguish it from FedEx. They are different model trucks, they make different noise.
*Is there something inherent to UPS truck noise that would make a dog (or just your dog) be excited or aggresive towards it? *
I doubt it, but I would assume if it was, it’s probably your dog just not liking the noise those trucks make. You know, finger nails on the black board type situation.
*So why does your dog hate the sound of a UPS truck? *
Most likely the sound formed an association with something negative. Maybe it had a bad experience with a UPS delivery man, maybe something unrelated bad happened while a UPS truck is passing by, maybe there is some sort of a pattern like if you get most of your packages from UPS when you’re not home, the dog can consider them knocking and leaving as an intrusion attempt that was stopped by it barking. As such, as soon as it hears the truck it starts pre-emptively defending the property.
The UPS trucks have something distinctive your dog picks up on.
Dogs differentiate sounds extremely well.
My Shih Tzu knows when i’m close to home by the sound, and ignores others unless they turn into the driveway.
I’m not a dog, but I can pick out the UPS truck by sound. Through the years, they have had a distinctive sound - like AskNott, I’ve always figured it to be due to their exhaust design. An article from 2003 says UPS has nine different engines in their package car fleet, and most are gas powered, rather than diesel, ranging from a 4.3 liter gas on up to Cummins 5.9 liter and International 444 V-8 diesels. Despite the different engines, they still manage to sound pretty much the same, and different from anything else on the road.
The translucent plastic roof is designed to let the daylight shine through so the driver can find the right package.
I worked for a UPS truck manufactorer in the early 90’s and can tell you that they have the same chassis that FedEx and Airbourne (now DHL) trucks have. Just different boxes married to them. That said, most UPS trucks were larger, had fiberglass roofs and had UPS spec cabins. UPS trucks were kinda special, and I have no doubt that they would resonat at a unique frequency that dogs could pick up.