The roar of a Harley is considered such a mark of distinction among enthusiasts that the company applied for a trademark on it! In 1994, Harley-Davidson, Inc. submitted the following claim to the USPTO, accompanied by recordings of their motorcycles with and without aftermarket mufflers:
The mark consists of the exhaust sound of applicant's motorcycles, produced by V-Twin, common crankpin motorcycle engines when the goods are in use.
Sound trademarks had been granted before, mostly on ad jingles or media/entertainment identifiers (the MGM lion's roar, NBC's three chimes). The Lanham Act establishing trademarks in the U.S. allowed for any "symbol" to be registered, and the courts have let registrants stretch the definition of symbol pretty far. This one raised a lot of hullabaloo because the sound made was incidental to the operation of an already-protected motorcycle design, and was similar to the sounds of other manufacturers' V-Twin engines. Opposition to the trademark was nearly universal, and in 2000 Harley-Davidson, Inc. abandoned their trademark filing.

Sources: The Trademark Registrability of the Harley-Davidson Roar: A Multimedia Analysis by Michael B. Sapherstein, Boston College Intellectual Property and Technology Forum, Oct 11, 1998
USPTO database at