The engine has been making a "da da da" sound these past couple of days. What could be the reason?

Call the after-sales service, and if that doesn't work, visit the service station.

If this issue arises right after maintenance, it might be an oil problem. The oil you add needs to match the engine. Another possibility is frequently using low-quality oil, which can lead to poor engine lubrication and eventually cause abnormal noises over time. Car maintenance is crucial, and I recommend going to a 4S shop for servicing. Outside repair shops, regardless of which one, might have counterfeit oil. It all depends on how much you're willing to pay. Good oil costs about the same as at a 4S shop, and at least getting maintenance there gives you peace of mind.