British inline sixes from 60's and 70's. Jaguar, Triumph, Austin Healy. All very smooth and mellow.
Any ducati V- twin thru a pair of conti's 1971-1984 stock standard, and the old triumph stags used to have such a nice burble.....again factory stock.
The sound of carboard on the bike wheel spokes!! adjusted with a stolen peg 
"AH " the sounds of child HOOD frivolity 
totally sick 
Is your favourite in here Dinsdale? Never flown in a DC3 but I did live close to the flight path of Essendon airport in the 60s, so I've probably heard more DC3 fly overs than most. A mate used to live right under the flight path in Airport West. They had a picture on the wall which used to oscillate as the planes went over head. When they took the picture down it had worn an arc into the gyprock.
I also worked with an old timer, he'd flown in New Guinea, who used to say " Look at this head " , it was a pretty rough head, "Been thru the windscreen of a DC3 three times". He also claimed to have looped a DC3, not sure if he was pulling my leg. Dinsdale?
Of the less exotic engines, I'd say the scream of an RD350 air cooled Yammie from the early seventies would have to be up there amongst the best.
