Can anyone identify the strange sound my bike is making?

That's called 4-stroking. Is the engine @ operating temp? Try leaning out your mixture. Also keep in mind that your rings haven't seated yet, so you're probably getting some blow-by. Also, break-in @ 25:1....then use 40+:1.

Yeah, there's a school of thought about the Chinese manual, and the quality of North American fuel products: Chinese petroleum products suck, so they use more oil. It's generally accepted that we can use 87 octane mixed with standard 2-cycle oil @ 25:1 for break-in. This may also reduce your 4-stroking right off. I'm running 45:1 on a broken-in 66cc, and it'll 4-stroke as much as yours until it reaches operating temperature. I'll ride it gently for a few minutes...like 1/2-mile, and it'll start purring. As for the choke, I assume you have an NT carb. My experience is, choke should be on (lever up) only when you need it. Once it turns over or starts, you can put it all the way off (lever down).

As for the rings seating themselves, that can take a few hundred miles; you should notice the engine performance improving over time...compression increased, as will power.

C-clip:
away from carb = lean (restricts jet)
towards carb = rich (opens jet)

For your reference, here's a recent ride...it was the first ride of the day, and it was about 32F. You can hear classic 4-stroking from 3:00 to 4:00 as the top-end heats up, then it goes into smooth 2-stroking.

http://www.youtube.com/watch?v=_Wb08gpTWdU

Ahhh, yes! Poor bastards waiting for the bus. Notice the the guy with the bike? The buses have bike-racks on the front. We had a severe blizzard a few weeks back and I'd relying on using the bus (and its rack) for my return trip on this same road. When I finally went to wait at the bus-stop, the roads were really nasty, and I was surprised to find the bus STUCK on the hill the stop was on. The driver told me she'd been stuck there an hour already, and expected to be stuck there for the night. The bike, however, handled superbly, and I made it home lickety-split. In fact, I had to negotiate my way around many abandoned cars.

I live in Connecticut where motorized bicycle riding is legal if you have a driver's license. I have however been spending a good deal of time on Long Island, which is New York State. Strictly, it's motorized bicycles are not legal, but the Suffolk County cops have been super cool...no probs at all. In fact, a lot have asked me details about my build. Another great benefit: I live in Bridgeport, CT, which has ferry-service to Long Island. Normally, it's about $110 round-trip with the car. They charge like $60 RT for motorbikes and scooters. They recognize me as a walk-on though, and I pay $24 RT. I tell people I wish I'd built a motorized bicycle 30 years ago (I built my first this past Summer), I was in college in Buffalo back in the '80s and went back-and-forth to Niagra Falls (Canadian side): 18 to drink and the strip-clubs went full-nude! A motorized bicycle woulda' been handy!

Do you have a 49cc or 66cc? If it's 66cc, you may be better off with the slightly larger NT Speed carb. They're available on eBay for less than $20 delivered. Also, what's your wheel-size and your weight?

I don't usually try much tuning until there're about 200 miles on it - things change till it's been run a bit. Even after that, it takes a couple miles each ride in 70degree weather before it warms up properly.

For what it's worth. I have never needed to re-jet a small engine carb. That's from 100" elevation, 100 degrees, 100% humidity- to 5500' elevation, 40 degrees, 20% humidity. I know alot of guys do, and you may need to, but try it all before you do it. I usually take the breather box off, and tune it a bit, then get it right with the air filter back on.
Good luck!

As others said, that sound is 4-stroking, you're probably running too rich, but if it's running decent it's probably best to run through a gallon of break in fuel and get on a standard mix before you do too much tuning, cause the oil/fuel mix will effect the air/fuel mix. Better to run rich than lean any how, a lean mix can run too hot and be hard on your motor....
BTW the e clip on the needle only really effects the mix in the mid throttle region. Moving the clip away from the pointed end of the needle will let the needle seat deeper in the jet and thus lean the mid throttle mix, however, this will not lean out the mix at WOT (wide open throttle). At WOT the size of the jet, the float setting and the resistance to air flow provided by the air filter and exhaust will have the most effect on the mix.
After you have put a couple hundred miles on the motor and the rings have seated and you are running a standard fuel mix (btw, I tend to break in with a gallon of 20:1 and then switch to a standard mix of 32:1 using standard 2-stroke oil, but not the marine kind) If you are still 4 stroking at that point you should do a plug chop at WOT (warm up the motor and ride WOT, then all at once pull in the clutch, drop the throttle and kill the motor) and pedal home. After the motor is cooled pull the spark plug and inspect it. Compare the plug to the excellent chart on this page http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html to determine if you are running rich or lean at WOT. At that point you take corrective measures but rather than speculating both ways right now I will wait for you to figure out which way you need to move....

Yes, I agree; before you make any big changes until you're fully broken in. On a oppositely related note: I was out last-night and had to fill up my tank. It was cold...about 25F...and as usual, I added oil to my near-empty tank, then speed-pumped the gas into the tank...shoving the nozzle against the oil-puddle which does a good job of mixing it up. Well, I guess the oil was too thick at the lower temps, and after I left the gas-station, the engine started running REALLY lean...nearly running away at idle, and head-temp shooting up to 475F. Turns out a glob of oil got into the carb-bowl. I've had the opposite effect, where too much oil in the mix caused the engine to stall. This is the first time I've had a "perfect mix" for it to lean out to the point of mimicking an air-leak. The problem resolved itself after a few miles.

once the oil all gets mixed even with a little gas, it will mix more easily with more gas - if you don't carry a small empty bottle with you, you can pull an old soda can out of the trash, wash it out with a bit of gas, put in your oil, shake it good, then pour it into your tank with the gas or before the gas

all the oil in the bowl might mean some amount of straight gas at some other point