A loud sound when shifting from 1st to 2nd gear on a new Yamaha FZ 25 could potentially indicate an issue with the transmission or clutch system. It's difficult to pinpoint the exact cause without more information or a direct examination of the motorcycle. It's worth having the clutch inspected to ensure it is properly adjusted.
Transmission oil: Low or contaminated transmission oil can affect gear shifting and cause noise. Make sure the transmission oil level is correct and that it is of the recommended type and quality.
Synchronizer issues: The synchronizers in the transmission help with smooth gear engagement. If the synchronizers are worn or damaged, it can lead to noise during shifting.
Gear engagement technique: It's also possible that the noise is due to improper gear engagement technique. Ensure you are fully engaging the clutch, releasing it smoothly, and using the gear shift lever correctly.
Since you mentioned that the first servicing of the bike is already done, it's advisable to reach out to the authorized Yamaha service center or a qualified mechanic to have the issue inspected. They will be able to diagnose the problem accurately and provide the necessary solutions or adjustments.
In some cases, a slight noise during gear shifting can be considered normal for certain motorcycles, especially when they are new. However, if the noise is excessively loud, persistent, or accompanied by other issues like difficulty in shifting gears, it's better to have it checked by a professional to rule out any potential problems.