VERY loud noise, need HELP PLEASE

amodoko /
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Hi guys, I have a sound problem that I need to fix but don't know exactly how. I have a '77 Sachs Columbia Commuter moped that was running just fine, but then I lent it to a friend to ride and it ran fine for him but when he revved the moped in place (he was sitting on it with his feet down and gave it some throttle, but he held the moped in place, I don't know why he did that) just for fun it started to make a very loud sound, kind of like a harley or like firecrackers are constanly going off. It hasn't stopped making that sound still. It still runs just fine, but it makes that "chopper" or "firecracker" sound whenever I am going slow (about 10-15 mph) or if it is in idle, and sometimes when I am going faster (25 mph). But mostly it just makes the noise when I am going slow or in idle. The sound seems like it is coming from around the cylinder head. It stops making the noise when I hold this little rod (it goes up and down on the outside of the cylinder head when the engine is on, I don't know what it's called cuz I don't know much about mopeds) on the cylinder head and prevent it from moving- but I think it's supposed to move, I don't know though. It also seems like the exhaust is still attached fine to the cylinder, but I guess I may have to take it off and reattach it to be safe (I have no clue). Also, there's a little hole on the bottom left side of the cylinder head, but that also looks like it's supposed to be there. I'm thinking something popped off or got loose, but I don't know what. That's about all I know for now, but I really need someone's help in getting this moped to be a little more quiet like it used to be. Plus, I don't know if that noise is bad for the engine or anything so I don't want it to take a beating. Please help me. Thanks for your time.

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