My car has had a rough idle for several months but it's not always very noticeable. Sometimes the RPM needle moves around a little and there is slight vibration and other times the needle drops down 2 or 300 RPM and there is vibration as well as some rattling noise. Recently, it was dropping down a lot when idling for about a week but as soon as I filled up with gas it went back to being barely noticeable again and I realized that I had been driving it with the tank below 1/4 for about that length of time. I think it's getting worse when I'm low on fuel.
The rattle sounds similar to a rattle I hear at 1500 RPM -not sure if it's related.
Anyone have any ideas?
Thanks

 

Important to know year.model,eng size, mileage and any recent maintenance history.
Different vehicles have different systems and variations can even vary between same year and model. The newer it is the more complicated it gets.
What some call a slight vibration can be an intermittent misfire, and that is why maintenance history is so important.
Newer vehicles have different fuel supply stages and fuel tank level is among the parameters used to gauge fuel delivery.
Evap systems can also get into the pix. so the more info you can provide ,is helpful.