the rattle could be your air con compressor or the condensor fan, when the compressor cuts in, the engine is ment to compensate the RPMS dropping at idle. this is usually done by a step up vacumm solenoid ( on carbs ) or the EACV on PGMFI. if you noise is a ticking metalic type sound then I hate to say your compressor has seen better days, unless you have recently had your air con serviced, thay may have over gassed it.