Don't listent to me, but I think it's just the normal vibration of the engine makes something clack in the engine bay while the whole block moves a little.

__________________
'81 M-B 240D 4-speed 132Kmi
'98 BMW 528i 5-speed/sport 125Kmi
'93 VW Eurovan GL 5-speed 166Kmi
'00 Hon RC51
'04 Suz SV650 track dog
'89 Yam FZR250R