Scooters
Scooters, mopeds, dirt-bikes. Generally anything on 2 wheels with an engine and is incapable of topping 50mph.
What a truly horrible invention.
My walk home from work takes me along the river front before finally turning off to the main road for 5 minutes. As I stroll off the tow-path I grumble at the noise of the cars and lorries trundling by at a mere 30mph.
Occasionally a Subaru or TVR will come by. You can hear them from behind due to the gentle rumble their engines make when ticking over in the city.
Other times you get mad-men on bikes where you hear the whine as they drop gears in a desperate hope to slow down enough to turn the corner. They have a nice roar to them, not many of them about to annoy you, and there is a type of music to the noise (not as good as say a Subaru IMHO).
Then you hear a whine. Ah, must be somebody mowing their law down that road. Then it gets louder and louder and ah ha. There is some council-estate dwelling college boy on his moped ragging it along the road weaving in and out of 1 tonne cars just begging to be run down.
Beeerrrrrrzzzzzzrrrrrrrrrreeeeezzzzzzz, they scoot on by with the strain of their 50cc engine screeching to move the fat kid on top. Annoying things. That's not the worst of it though. After it's gone past, and the insanely loud ferrari has gone past too, and the articulated lorry, and the (etc). You can still hear the strain of this bike a good mile and a half on down the road.
Such a truly horrible noise. It's there with toddlers screaming at the top of their voices. The scratching noise you get on blackboards or on the dinner plate when your fork slips.
Luckily this has been resolved by a bike that is powered by a fuel cell. Making it almost completely silent. It is also environmentally friendly (so much you can drink the exhaust fumes). Details by the BBC:
http://news.bbc.co.uk/1/hi/world/europe/4353853.stm
Shame they are deciding to put a fake engine noise on it. It's going to be some nasty screech from a tiny speaker in it that can't produce a real noise just an irritating thing in your ear.