I bought classic 350 about 8 months ago but since after only first 15 days of riding i started noticing tak-tak like noise from nearby engine. Noise can only be heard while riding on low rpm when i accelerate after changing gear. But on neutral that noise dissappears. I visited royal enfiled service station but their mechanics cannot hear that noise and told me that my bike is ok. I have complained every single time while servicing my bike about that weird noise but they could not hear and understand what i have noticed.
There are two potential issues
You have too much valve clearance and the sound you hear is valve related. This is a typical maintenance item and can be rectified by adjusting the valves. The plate below your pushrods can be removed and the adjustment is completed there.
Your are hearing your piston - Air cooled motors have extra piston clearance between the cylinder and the piston due to increased expansion due to more heat. Air cooled engines tend to run hotter. On many older designs you can hear the piston slapping within the cylinder as it rocks back and forth on the wrist pin. This is considered normal.