What shud i do then rustjunkie
can't fix it if we don't know what has happened, what the noise is like and when it happens.
gotta know the history and see the parts to determine what the cause of the problem is before a solution is suggested.
what does the noise sound like?
does it happen when the bike is pedaled gently in a workstand or not?
does it happen when pressure is applied to the pedals while riding?
as we learned here:
Maintenance of bicycle chains.
www.sheldonbrown.com
chains and cogs wear together.
a very worn drivetrain will get you down the road well-past it's "acceptable" wear point, not making any noise and not "skipping" if the rider doesn't put enough pressure on the pedals.
BUT! change the cog or chain and then there's trouble:
Maintenance of bicycle chains.
www.sheldonbrown.com
have the cog and chain been recently introduced?
you've said the cog and sprocket are pretty much new...let's see pictures of the cog.
most don't know what a worn cog looks like; no fault: we don't know until we know.
fordmike gave good advice about the chain: they wear out (not stretching really) effectively changing the pitch, as noted in the sheldon brown link above.
is the chain nos?
used?
what does it measure and how many links does it have?
nutshell: as the chain and cog wear, the pitch is changed.
introduce old to new and noise comes from the struggle of the components to mesh with each other.
c'est la vie