Sounds like my high-school ride.  1965 Ducati 250 Scrambler.  They didn't come wit' no muffler.  Closing the throttle sounded like it was sucking air back into the exhaust pipe.  A friend also had one.  We stuck a little single baffle shorty muffler on them so the cops couldn't stick their muffler-test rod up the exhaust pipe.

Sounds better than my favorite guitar! I wish Triumph would make a 500 single.