If the noise goes away when the clutch pedal is pressed down, then the problem is most likely the clutch throw out bearing.
It may be the stainless steel shrouds on the exaust pipe. My 1994 truck will rattle when the engine is at a certain speed. The screws holding the metal together have begun to rust out, and the loose pieces are making the rattle. My 1994 truck does just the same. About 3 years ago I was advised by the dealer to replace the timing chain immediately for about 1600.-$CDN (typical advice for a female...) and shrugged it off ever since. I just replace the engine oil more often. I read a few times that the models around 1994 have a slightly lose timing chain causing the intermittent rattle but are of no major concern. I will post again if my engine blows up after all....
Perhaps clutch throw-out bearing?
the clutch
Clutch throw out bearing bad.
The fan clutch is not functioning properly. Replace.
The clutch bearing or " throw out" bearing is going bad and will need to be replaced, take this opportunity to change the clutch as well.
Yes, "sparkling plugs" is definitely detrimental to idling or smooth running of an engine
check to make sure that you have the right amount and type of transmission fluid, that it is in good condition not dark or burnt smelling, if so change. change fluid and clean filter.
it could be your timing is off
Check your spark plugs and leads.
When your standard car is idling, it should be in neutral. If it isn't, that means you are holding the clutch in which will eventually burn it out. Save yourself some time and money and only use the clutch to shift. Otherwise keep it in neutral when you are idling.
Vaccuum leak somewhere
A bad exhaust will cause gas fumes to get inside the Nissan Maxima while it is idling. Gas fumes, if contained in an enclosed space, is dangerous.