Whether it has been modified depends on whether the screws have been tightened or not.
You can also check the production date on the gasoline stick to see if it is close to the production date of the vehicle. If it's close, it means it hasn't been replaced. If it's too far apart, it means it has been replaced.
There are two main reasons for the loud noise. 1. The connection between the motor crankshaft and the connecting rod is worn, resulting in a gap, which will cause noise. This problem can be disassembled and a small amount of paint should be applied to this part to remove it from the machine. Grease lubricant (around 40#, solid) is enough;
2. The fixation between the motor and the casing may be loose, or the isolation glue may become hard. If the oxygen pump works continuously for a long time, Heat will be generated, and the heat will accelerate the hardening of the isolation glue. To solve this problem, you can put a layer of transparent glass glue between the motor and the casing. It must be placed for 12 hours before use, otherwise the effect will not be good. Don't use Gelianghao glue (AB glue). This kind of glue is hard and the noise will still be loud after gluing. The glass glue is also very elastic after it dries.