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I have a Nissan pulsar ls hatch which has done 208000 km. it has a noise that gets worse as the engine heats up .This goes away as the engine revs,Could it be a belt?

It's very difficult to say what this noise might be without hearing it. However, as it gets worse with increased temperature and only evident at idle, it doesn't sound belt-related (but anything's possible and it could be).
A couple of things that spring to mind as possibilities are perhaps an air leak on the intake somewhere or perhaps if it's a manual, maybe a clutch release bearing???
However, I can't really rule out anything (belt, mount, bearing, water pump etc, etc), so really, you need to have a mechanic listen to it and give you an opinion.