Triumph Engine Noise

I'll confess, I'm only 23. That's not necessarily the problem, however I can say I haven't been around long enough to be able to listen to the Triumph in my barn and tell you if it sounds like it should.
It started fine a few days ago, while running it makes a chattering metallic sound... did the ol' screwdriver to the ear trick and thought perhaps the valves slamming open & closed? A strange rapid 'tinking' or chattering is the best I can describe it.
My buddy rode it for a few days, then came cruising in the driveway claiming he had lost a cylinder. Cleaned the carbs (weird clear spherical build up? ...no idea where it came from or what it is as the engine was recently professionally rebuilt)and they bleed beautifully now, went over everything else in the area... Now she won't start. Kicked for 30 minutes, bump starts, everything.
Today my buddy plans to get it started and seek the advice of (older) friends who know what the thing should sound like. Thought I'd ask around here too. I know this is a little vague, hopefully one of the local Triumph guys gets a listen to it today. Any ideas here?