Thanks for comments (Imran Bhatti, Ahmed Raja, Vickytulla, Metalhead and Xulfiqar); serial wise replies are as:
Noise starts in the beginning and increases as engine gets hotter, sometimes it feels like tappets are 'Free' inside the head.
Previously used 'Havoline 20W50' normally used for cars. Recently changed to 'Kixx 20W50'. Interval of change between 1000 to 1200 kms. Can you suggest proper oil for this engine? No idea about factory specs.
Performance decreases, fuel consumption increased.
Overheating is never felt. But sometimes decrease in efficiency is noticed.
No idea about 'lower rockers'.
Actually no 'proper mechanic/workshop' is known to me in Rawalpindi or Islamabad. Shown to many, but no proper diagnosis. Confused at the moment, no idea what to do. Sound is increasing everyday...:S