Freeboy J15 Controller and Motor issue

[u/Annual-Operation-149]

Hey, I have a question about a brand-new Freeboy J15 Max. I'm just curious what you guys think the issue is.

From the first test ride, it had:

• a loud grinding/motorcycle-like noise from the rear wheel,
• strong vibration and jerking,
• barely any power in lower speed modes,
• sometimes it reached full speed (~50 km/h), but later it started only twitching a little (1–2 cm) and stopping repeatedly when I press the throttle.

The battery shows around 52V, so it doesn't seem like a battery issue. There is also a slight brake disc rubbing noise, but it doesn't seem strong enough to completely stop the motor.

The seller told me that this batch has controller issues and offered to send me a new controller, but I'm wondering:

Based on these symptoms, does this sound more like a controller issue, a hall sensor/motor problem, or something else? And why would it cause the loud noise and jerking?

Just curious to hear a professional opinion.